I haven't read many books, but I would have to say "The Lost Years Of Merlin" by T.A. Barron. It a five book saga about Merlin in his youth, from the age of 10 to 18. The books are, The Lost Years of Merlin, The Seven Songs of Merlin, The Fires of Merlin, The Mirror of Merlin, and The Wings of Merlin.
I first encountered it a many years back when I was in high school. I didn't really like reading that much, so obviously I hated doing book reports. Normally, I would just grab a book and just read a few pages in the beginning middle and end, so that I could do the report. But at that time, I decided that I would at least look for something interesting. I found "The Lost Years of Merlin" in my school's library. I had intended to do what I normally did, but after a few pages I was hooked. It was the first book report that I actually read the whole book for. Then I found out there was more to the story and I went out and bought the five book saga at a Barnes & Noble.
Each book is just under 300 pages. Since I was normally didn't read books, I was surprised by how six hours would easily fly by while reading one of them. I finished all five within a month or so. T.A. Barron has also written a trilogy that takes place after those books, The Great Tree of Avalon. I finished the first book and I'm reading the second. Most of his books involve around the time period that Merlin lived. When I finish the series I'm on I will probably move on to his Merlin's Dragon series. The guy is really prolific writer.
Really, thinking back, his books are what got me interested again in writing like I had been in elementary school, and is why I became an English major in college.