Comment Pocket Ref (Score 1) 517
http://www.amazon.com/Pocket-Ref-Thomas-J-Glover/dp/1885071000
'Pocket Ref' is a conveniently sized book containing an absolutely outrageous amount of data. In 3-3/4" x 5-1/2" x 3/4" dimensions and around 500 pages, Thomas Glover covers topics from ASCII tables, to load bearing capacities of 2'x4's, to a comprehensive math and physics formula 'cheat sheet'.
I don't know if it is possible to exaggerate how useful this book is. Along with a decent calculator and a knack for solving practical problems, you will be unstoppable with the Pocket Ref at your side. McGyver certainly had a copy hidden in his shirt pocket.