The earliest I can remember apart from the trivial Janet and John stuff which I went past really quickly, would be the Famous Five books. I think I started with the Secret Seven, read one or two of those then most of the Famous Five when I must have been about 5 or 6, and other Enid Blyton especially the Adventure books
Treasure Island
CS Lewis's Narnia books
HR Haggard: King Solomon's Mines, She
Biggles
Reach for the Sky
Willard Price's "Adventure" books (from the library)
I read some Mark Twain, can't remember which one.
Jack London: Call of the Wild and White Fang.
Tolkien: The Hobbit. I never read LOTR until adulthood.
Edgar Rice Burroughs' Tarzan books, and the John Carter of Mars series
Star Wars (I got the novelisation from the school book club* and read it about 5 times before I got chance to see the film! I've just googled it as I thought it didn't sound right given I saw the film pretty soon after it came out, but yes the novel was published in late 1976.
My aunts were heavily into scifi and fantasy so I could borrow a seemingly limitless supply! The Conan books by Robert E Howard (and later L Sprague De Camp). Michael Moorcock's Elric and Corum novels, Asimov, Heinlein, Alan Dean Foster, Harry Harrison, Poul Anderson etc.
One of the best Christmases I remember was when I got a stack of books including Treasure Island, one of the Narnia series and one of the Tarzan ones. The smell of a new paperback still takes me right back to then. I must have been about 6 because I remember my teacher making a fuss because I was reading "such an advanced book" for my age (it was Voyage of the Dawn Treader). I didn't get what the fuss was about, I thought it was quite an easy book, but they told my folks I had a reading age of 16.
*The "club" where every month or so you got a printed leaflet with a selection of books and could tick the boxes next to your choice(s) then fill in the rest of the form with your name etc. I remember the excitement when the stack of books was on the teacher's desk with the order slips wrapped round the books.