Not counting “Dick and Jane” and its school-assigned equivalents, what is the first book you remember reading as a child?
Mine was The Wonderful Flight To The Mushroom Planet by Elinor Cameron…quickly followed by everything else I could find that was written by her.
You?
The first book that wasn’t primarily a picture book was Tales of Toyland by Enid Blyton. I still have it.
The first book I have a vivid memory of reading was the second book in the Nancy Drew series, The Hidden Staircase. Before that I had been prompted page by page to read, parental pointing etc; in that vein I “read” books like Mike Mulligan and His Steam Shovel and Stop that Ball, The Cat in the Hat Comes Back, Spaghetti Eddie, etc. But my Dad read some Hardy Boys and Nancy Drew books to me, one chapter per night, got me hooked, then said “I’m sorry, I’m too busy this week, but you could try reading ahead on your own” and handed me The Hidden Staircase. Sneaky Dad.
Hop on Pop.
The Shark In Charlie’s Window when I was around 8 years old, I think. I was a good at reading as a kid, but not a book-reader as such. I read comics and picture books and puzzle books, but not novels.
There are a lot of classics I have never read, even though we had them on our shelves. Treasure Island, Famous Five, Narnia, Wizard of Oz, Tom Sawyer, etc. Their antiquated style was a bit much for me.
The first book I remember reading was Witches, Witches, Witches. I couldn’t have been more than four years old. Scared the crap out of me! 
The second was probably All About Dinosaurs by Roy Chapman Andrews, followed by the Classics Illustrated versions of The Time Machine and War of the Worlds.
There was one more, published by the same company that did Andrews’ book. It was about great explorers, and there was a chapter about the dive of the Trieste to the Marianas Trench. Can’t think of the title though. 
Let’s not start this again.
I think it might have been Beezus and Ramona. As I recall, we read a story from the book in class and I enjoyed it. So when I saw the book in the school library and realized there were more stories with the same characters, I read it.
Astonishingly, the author Beverly Cleary is still alive.
I can remember reading The Wizard of Oz when I was a child and being astonished to discover the story in the book was different than the one in the movie. I guess I had assumed that there was only one way to tell a particular story.
Little House in the Big Woods, by Laura Ingalls Wilder. Followed shortly by Aku-Aku by Thor Heyerdahl and The Hobbit. Some books on pond life and other science titles in there as well, and Scholastic books including one about how to write cyphers.
Beastly Boys and Ghastly Girls It was a collection of poems by famous authors.
Nothing to do, nothing to do,
Put some mustard in your shoe,
Fill your pocket full of soot,
Drive a nail through your foot…
Winnie-the-Pooh. Or perhaps The House on Pooh Corner. Definitely one of the Winnie-the-Pooh books, in any case. It definitely wasn’t the first book I read on my own (I was a very early reader), but it’s the earliest I can remember.
The Best of Science Fiction, edited by Groff Conklin, at the age of 6 or 7.
It was a collection of 40+ SF short stories, and I pretty much read it from cover to cover. I’m not sure how much I understood some of the stories, and I found some of them disturbing, but I really loved that book.
How the Sun Was Brought Back to the Sky by Mirra Ginsburg.
This book seems to have prompted me to question my dad about what happens to the sun when it’s cloudy out. It was an overcast day and my dad was inspired to drive me up to the mountains and show me that the sun was still there above the clouds.
The Story About Ping by Marjorie Flack is another one I remember… maybe it was the first one.
I should note that both of these books were already quite vintage by the time I was born.
I had lots of books read to me as a child. My SiL tells me when she came to see my bro, I proudly announced that I could read the newspaper (at age 5). She didn’t think I could but yep, I sure could.
Because the OP says “Book,” I’ll rule out comics. Yeah, they’re comic “books” but they’re comparatively short and they’re illustrated. Similarly, it may have been an anthology by Edgar Allan Poe (not illustrated)—“The Black Cat” is a great short story—but also, I didn’t read the anthology from start to finish.
I think one of those Bookmobile things came around and I got “The Great Brain” by J.D. Fitzgerald. It’s an assortment of stories, but I read it all. Our teacher was pushing reading and my parents bought me a book, which wasn’t usual.
In fact, the teacher asked someone to make “medals” for best book reports in various categories and I somehow ended up doing it (by which I mean outside school). The idea was that after we presented the reports, the class would vote and awards would be given. I had powder blue construction paper, glue, tubes of glitter, scissors, a paper punch, and some yarn (to make a “necklace” for hanging it around the winner’s neck). There were categories for the awards, like best adventure story, best action story, etc. Then there was the grand daddy: best book report.
I made all the awards but that award, for the best overall, didn’t turn out very well. You know, you try to handwrite with glue but it doesn’t always turn out. Small, messy, unimpressive.
Yeah…the best book report was the one I won. At first I was kind of bummed out. Then I realized that since I had made them, I could make another, better one. Then something clicked and I matured by like, ten years or something.
My Grandma had one of those Time-Life books, a large but thin hardcover, that was a picture and diagram summary of the Mercury-Gemini-Apollo programs, and the Apollo 11 Moonshot. I loved it as a kid, mostly for the pictures, but I did eventually get around to actually reading it as well.
The first book I ever owned, that I picked out for myself, was H.M. (Helen Mary) Hoover’s The Delikon.
Still have a copy.
The first novel I read, at the age of 7, was either The White Mountains by John Christopher or A Wizard of Earthsea by Ursula K. LeGuin (can’t remember which came first).
All About Undersea Explorers, by Ruth Brindze?
… Famous Scientific Expeditions, by Raymond Holden?
There are a few dozen of those books, published by Random House; my sister bought me several when I was around six or seven. She had a subscription – one “All About” book and one “Landmark” book each month. The earliest books I actually remember reading were Profiles in Courage* (JFK) and The Witchcraft of Salem Village (Shirley Jackson), both of which were “Landmark” books, though I’d already been reading for several years by that time.
* According to my cousin I was seven when I read Profiles in Courage.
Too late to edit: Here’s a complete list of the “Landmark” books.
There are lots of books I remember reading as a child, but “what was the first” is a hard question. I have a vague memory of finishing a Bobbsey Twins book (no idea which one) and thinking “I just read a book all the way through!” so that might have been the first novel-length book I read.