Klutz LEGO Books – Klutz has a line of LEGO books that illustrate simple mechanical concepts that may be generalized into larger personal projects.
The LEGO Technic Idea Books – Yoshihito Isogawa’s three magnificent wordless books of LEGO Technic project ideas are like the holy books of LEGO construction. There are enough ideas contained within to keep you building for years!
The LEGO Power Functions Idea Books – Two volumes by Yoshihito Isogawa. Volume 1 showcases projects that explore mechanisms. Volume 2 covers cars and other moving contraptions.