If you are looking for a couple of GREAT pocket-sized engineering resources, I highly recommend you add either/both of these to your toolboxes.
I picked these two books up in the early 1990's and they are as useful today as they were when I bought them as a young upstart engineer. LOTS of simple, basic general engineering & mathematical formulas that we all use during the workday are contained in these pocket-sized books. Although we use computers to find information & produce calculations moreso now then we did back in the day, it is always good to have access to the basics for reference. Cheers!!