Combining useful physics lessons, building cool machines, and LEGO is always win in my household. This book has my daughter back to building!
LEGO Chain Reactions: Design and build amazing moving machines arrived as a gift the other day. This very clever book easily walks you through building Chain Reactions, simple LEGO machines that are designed to modularly fit together, they can function individually or as a unit. There are 10 models that you can build and link together.
In addition to each module being a interesting mechanical construct, the book explains the physics that allow the "chain reactions" to occur in a way my 8-year-old gets it. There is also a useful progression through the book in a "Now that you've built an onager, meet the trebuchet!" sort of way that I found helpful and entertaining.
At first I was worried that our vast tub of LEGO would not have the right pieces. If you have 2-3 sets worth of LEGO detritus, you should be fine and able to build everything here. The included pieces are all specialized, but if you follow the books instructions, and don't care about color matching, you'll get there.
IF you want to build some fun RubE Goldberg machines with your kid, LEGO Chain Reactions is a great book.