I was a voracious reader when I was a kid, literally hundreds of books a year. Now that I'm more mature it is interesting to go back and re-read what I thought was prime literature at the time, or even read a book I wanted to but didn't back then. Since they are written for kids they take hardly any time to read.
One of the latter category is Lloyd Alexander. Best known for his Prydain Chronicles series he also wrote a series of adventures featuring Vesper Holly as well as many others. I must admit to enjoying them. The Vesper series is humorous even today with its historical references to Victorian mores and custom but with a very modernist tilt.
To Kill a Mockingbird was my favorite book and, I am pleased to say, has passed the test of time. It is still my favorite. Another set that also passes the test of time is anything by Ruth Chew. I highly recommend them to any child who is interested in history and adventure.
One that I was not interested in as a youth and still have not plunged into but was incredibly popular is the V. C. Andrews series. I do have a friend who has recently re-read many and she says they still hold her interest even almost 30 years on.