I've been reading Anno's Math Games with ds6 and he loves these books (there are 3 I think). They have various puzzles, mazes, "visual riddles" (if that makes sense), etc. The House that Math Built is good too - dd3 likes that one too. They are both very visual - lots of things to look at, questions to ask, things to figure out. Math concepts are introduced in a fun way.

Does anyone know of any other books like this that encourage critical thinking for early elementary?