In responding to this prompt, I am rather hampered by not remembering books particularly well, and having read relatively few of late. I wasn't sure that I felt strongly enough about enough of them to rec in any way, or could come up with 10.
But as it turned out I did find more than 10 books I've found memorable, though as I haven't read any of them in a very long time I don't know how well they really hold up.
In putting together the list what seemed significant to me is that none of these books stand alone. What I clearly loved even more than genre was seriality. Given my longtime fannishness this perhaps goes without saying, but it's what leaped out at me when I began looking at my bookshelf. Second is that I love historical fiction as much as mysteries. And third, that these books serve as a sort of biography. As such I decided to put them in chronological order.
1) King of the Wind by Marguerite Henry. I read all the horse books I could get my hands on as a kid, such as the Black Stallion series, biographies of famous race horses, etc. I hung onto this one for a very long time and loved it best, perhaps because it was historical fiction and a pretty interesting story at that. It's a shame there were no dinosaur novels back then or I'd have eaten them up. I do recall some sort of How and Why book on them which I knew by memory but it was a science book not a story.
2) 101 Dalmations by Dodie Smith. This one stands out for several reasons. For one, I think I bought this through the Scholastic Books reading program in grade school. Getting that catalog in class, having time to review it for books I might want to read, was always one of the best parts of school. It might have been the only good part of school, since I can't recall anything else I enjoyed. And the day in which the book shipments arrived! Better than Christmas.
But returning to the book, it was among the first full length books I'd read. Even though it's a children's book it was pretty complex for such a young reader, as I was reading the novel itself and not a Disney adaptation. Also new to me was the English life depicted and English terms which I had no frame of reference for. But the tone of the book always stuck with me. It was perhaps the genesis of my Anglophilia.
Another childhood classic I recall (and re-read many times) was Peter Pan by Monteiro Lobato. It was in Portuguese, I never read the Barry version. What I most remember about that now was that my mother tried to stoke my interest by getting me other Lobato books and I read his O Saci. I found it horrifying and still remember a gruesome scene of a character being killed by being buried alive in an anthill. No further books by Lobato for me, and definitely a less successful introduction to another culture's literature.
3) Nancy Drew and her many friends, by which I mean the Hardy Boys, the Bobbsey Twins, the Dana Girls and a few Tom Swift books. But really the others I read because I'd run short of Nancy Drew titles from the library. I can't recall how I first got directed to her, though I am pretty sure it was an elementary school librarian who suggested them during 4th grade. I'm not sure if I'd already been interested in mysteries somehow or if it was reading Nancy Drew that did it, but it's been my favorite genre ever since. And it was a series of books in which a young woman and her friends were the heroines, the problem solvers, and smart and independent. By contrast I don't recall ever reading books about princesses.
4) Murder on the Orient Express by Agatha Christie. I've actually read all of Christie's works and I think that she is likely the author with the biggest influence. This was the first book of hers I read. I recall that I was 10 and traveling with my parents. The book was a re-release with a cover from the 1974 movie poster. I knew only a few of the actors in it and this was pre-Internet so I spent some time trying to figure out which actor represented which character since this was pretty unclear other than for Poirot and the young Comtessa.
I remember I had gone through all the books I'd brought on the trip (ah, how I love you eReader!) and my mother suggested I try Christie as she was a very famous mystery writer. I don't think we were in a bookstore but a newsstand or gift shop, so it was on a paperbacks rack. I remember being utterly fascinated with the book which was my first "adult" mystery and is, of course, a classic for a good reason. I remember when I did see the movie I found the scene that recreated the murder rather creepy but I also recall thinking that Albert Finney was not quite right as Poirot.
In a digression, I also realized early on that horror was not for me. I don't recall exactly when I read The Exorcist though I'm guessing I was in middle school or maybe high school. But I remember being so freaked out by it that I don't think I even finished it. Years later I had a less strong reaction to Stephen King's "The Shining," though perhaps I'm now misremembering and it's actually the movie that made me temporarily afraid of the dark. I have in fact read a number of King's books but the closest I came to reading another horror-type novel was Audrey Rose by Frank De Felitta. I quite liked that one, though it's more a supernatural book than a horror one, which is also the attraction I had to the King novels.
However, that definitely made me read every other Christie novel though it took many years before I went through them all. Thankfully she was also prolific! I remember realizing I was getting better at figuring out mysteries once I was able to figure out her plots, though these were relatively few of her many stories.
5) The Making of Superman by David Michael Petrou. This was released in 1978 coinciding with the movie and I read it then. It was my first introduction to film-making or, really, any serious behind-the-scenes story about entertainment productions. For a while I became interested in working in that industry. While I'm not sorry I didn't, I developed a lifelong interest in not just stories but also how those stories were made, particularly huge productions like that undertaking.
Relatedly, the period from 1976-1979 was also a time when I was reading comics –- DC as it happens. This had more to do with the Super Friends show than the comics themselves or even the Superman film. But along with the movie came the novel Superman Last Son of Krypton by Elliot S. Maggin. This was not an origin story but an original novel. I haven't read it in many years but recall Superman and Luthor had to team up together to handle some mystery involving another planet that they went to together. It was a case of fanfiction and also eye opening to me about the depths at which stories could go, even if they began in comparatively simple comics.
7) Speaking of fanfiction, I'll list Diane Duane's "Wounded Sky" next. It was not the first Trek novel I'd read and perhaps not the most influential in that it didn't introduce me to fanfiction (that would be Star Trek Lives). But the earliest tie-in novels, while satisfying in that they gave me more Trek (my first real fandom) were not stories that enthralled me. Duane's may not have been the first either, I know that there were others like Vonda McIntyre's "The Entropy Effect" that also grabbed me. But she had several Trek novels which I enjoyed. And I liked that one well enough to recommend it to people. I read Trek tie ins for all the shows on at the time, though by the time the Next Gen books began, the books were being produced at such a clip that they became increasingly formulaic and eventually hardly edited at all (or so it seemed).
8) Mists of Avalon by Marion Zimmer Bradley. I remember my closest friend and I were reading this together. We were both completely hooked and it cemented an interest in all things Arthurian. Of course, I'd already been interested or I wouldn't have picked the book up in the first place. While pretty much any book based on a myth is fanfiction, this one rewrote the common tales in a very particular female-centric way.
As with Christie, this book kicked off a variety of related reads, though not of Bradley. I tried her book about Cassandra and was quite disappointed in it. It seemed a pale echo of Mists, and I never read anything further by her. I did, however, read a number of other books which could probably rank in a top 10, such as The Crystal Cave by Mary Stewart or Firelord by Parke Godwin (and their sequels), Sword at Sunset by Rosemary Sutcliff and The High Queen by Nancy McKenzie. There was a book written by an academic, Nathan Comfort Starr in 1954 that attempted to compile all the modern novels based on Arthur (King Arthur Today: The Arthurian Legend in English and American Literature 1901-1953). It was already a long list of books. I can't imagine what such a list would look like today, though people are trying. The most recent Arthurian book I read was last month.
9) Hot Money by Dick Francis. I can't recall who or what first recommended me to Francis, though combining horse racing and mysteries into one, his works should have been made for me. I read them all until his death, I think. I found them somewhat frustrating, sometimes in the attitudes of his characters but almost always in their endings, which were abrupt and left me wanting for closure. I single out Hot Money because it was one of the first books I read of his and also focused on the Breeder's Cup, and even takes place in part at Santa Anita Park, both of which I've been to.
10) The First Man in Rome by Colleen McCullough. A hefty novel about the period predating and including Julius Caesar, I remember being struck by how little humanity has changed over the centuries. Its societal structures, its conventions, sure. As people, not a whit it seems. I read several of the sequels but never got around to the fourth book because of time constraints and then it simply sat. It's still waiting for me on the shelf. Along these same lines I should mention James Clavell's Asian saga, which I was once current on.