Does the HM book have pictures of the HM or just directions on finding them? It would be fun to have a kids book...like the I Spy Books, that have real pictures of the HM that you have to find! My son would love that!
The book Hidden Mickeys, by Steven Barrett gives brief descriptions of what type and where they can be found. No pictures. Some are easy, some are hard. Sorry to say, I wouldn't consider it geared towards a 5 y.o. on his own. It was fun and sometimes a challenge for our family.