There is a whole series of these books. They are nothing special for pictures of and facts about machines. What is great about them is that they each have a make-it-yourself and a trace-your-own page in the back. There are instructions for making trucks from old boxes and bottle caps. We haven't tried any of those yet. Maybe later. The trace-your-own pictures are clear and bright, good for copying and coloring.
Lots of pictures. Clear, simple explanations of how diggers work. The projects at the end (make your own digger using cardboard boxes, etc.; and trace a digger) are a bit old for my two-and-a-half year old. But I can see that they'd be appealing!