Still Missing by Beth Gutcheon is one of my favourite books:
Alex Selky, going on seven, kisses his mother goodbye and sets off for his school that's only two blocks away. He never gets there. His mother is desperate to find him and begins a vigil that lasts for days, then weeks, then months and never gives up hope that he'll be found alive.
This book originally came out in the early 80's but had been out of print in Britain for some time before Persephone books published it again. I wasn't sure how I would get on with reading it as my daughters are a similar age to the missing boy but I'm so glad that I did. As the story progresses and the weeks turn to months with still no sign of Alex, his mother Susan is the only person who believes he's still alive somewhere and her family and friends feel that she should start to try and move on. This is a very well written book (which you would expect if Persephone have published it), I couldn't put it down and I was desperate to find out what had happened to Alex. I originally read this just after it came out and ended up taking a longer lunch break at work so I could finish it.
One of the best books I've read and certainly one I'll revisit.