Lost Boy Lost Girl tells the story of a teenaged boy, Mark, who becomes obsessed with a certain house, and then disappears; the lost girl is harder to explain, but woven into the book deftly. All of that is informed with the knowledge that there's a serial killer at work in the town where Mark lives. The reader follows along as Mark's uncle investigates his disappearance, and the events leading up to it. Straub keeps things interesting with shifts in the timeline throughout the novel, following one thread for a while and then going back to another, so there's a sense of the way the developments overlap. Because of that, the reader gets to connect with Mark along the way, while knowing that he's disappeared.
Okay, I'll name-drop a little. I met Straub at World Fantasy, and was in the middle of reading this book when I got him to sign it. I said that I was looking forward to seeing how it ended, and he told me that it had "kind of a non-ending." Now that I've read it, I see what he means, but I liked being able to form an opinion of my own about the ultimate fates of the lost boy and lost girl, so I don't see the (non-)ending as a bad thing at all.
I'd recommend this book for nearly any mature reader, because it's simply a good story; there are things about it that could be really alarming for some, but I hope that people can read the cover copy on a book and decide for themselves whether it's too scary for them.