I very strongly second both suggested readings, if only because I have both (including the complete set of Narnia books), have read both, and find them to be examples of some of the best fiction writing known to mankind. I would be somewhat less of a person if I had not read those, I think. With regard to the Narnia series -- the books were written and published out of order. See here --> https://en.wikipedia.org/wiki/The_Chronicles_of_Narnia#Reading_order (I recommend the "Harper Collins order" of reading the books, as referred to in the linked article.)

The Phantom Tollbooth, I will warn you, is a "Young Adult" novel -- read it anyways, it's a wonderful story and a wonderful book.

I can also recommend some of Brian Jacques' earlier works (the Redwall series of novels) -- I should warn you that the later ones get very predictable -- my favorite from his would have to be The Bellmaker. By the way -- if you're into sci-fi -- I could do far, far worse than to recommend Coyote by Alan Steele and Eternity Road by Jack McDevitt. Coyote is about colonizing another world -- but it's far more (pardon me) "down to earth" in how things work out, both in terms of character development and technology, than, say, Star Trek. Eternity Road is a post-apocalyptic book -- one of only two, honestly, I've really met that I've enjoyed (the other is called The Rock -- referring to Ayers Rock in Austraila, which is all I'll say about the plot) -- although I have to say I can't really recommend any of Jack McDevitt's other works. They... I dunno, they just don't 'click' for me, you know? If fantasy is more your thing, there's Anne McCaffrey's Pern series -- I loved those books when I was in high school. Absolutely ate them up. It's sort of a sci-fi/fantasy fusion sort of a setting, but it works quite nicely... I'll not explain what I mean by that, though, because that spoils major parts of the plot in one of the books...

Of course, I would also be quite remiss in failing to recommend the first three books in what Wiki wants me to believe is in fact a quintet -- Madeleine L'Engle's A Wrinkle In Time, followed by A Wind in the Door and A Swiftly Tilting Planet. (I have neither read nor even heard of, the two books that are supposed to go after those three, titled Many Waters and An Acceptable Time -- so I can't speak to those. Sorry.)

...you can probably tell already, but books and reading are valued things in my family... books are amazing things. They can -- if you let them -- take you all sorts of wonderful and strange places... I'm happy to recommend even more if you're interested -- shoot me an email with a topic or fiction genre and I'll gladly see what I can recommend. (I should note that I habitually stay away from horror and similar material at all cost, though, so if that's your thing, I can't really help.)