I just finished the first book in The Chronicles of Narnia, titled The Magician's Nephew, by C. S. Lewis. From Wikipedia, I learned that it was actually the 6th book written. By its "internal chronology", it is first. I totally enjoyed it. It set up The Lion, The Witch, and The Wardrobe wonderfully and answered some questions I had while reading The Lion. . . .
The book raised another question though, like "What happened to the tree?" You'll have to read it to find out what tree! The answer is probably already there though. It is intimated that it will only last for a few hundred years to protect Narnia from evil. The Lion, The Witch, and The Wardrobe doesn't happen until quite a long time after The Magician's Nephew takes place.
More importantly, I discovered how to entice my sons into reading it. I put it on the coffee table after finishing it, exclaiming over it, and Dragon promptly picked it up. I think that book will stay very much in sight! Once he's done with it, I have no doubt my other son will be interested in it too. Right now, he is engrossed in the Tennis Shoes series.
P.S. I'm up to 36 books so far this year! Woot!