Just my general bookshelf but...
For general use I like to keep a hard copy of CRC Handbook of Chemistry and Physics. (Wistful recollections of the same publisher's Handbook... Engineering Sciences.)
Another good CRC book I have is the Standard Mathematical Tables and Formulae.
I recommend any edition of Resnick and Halliday on Physics.
I spent lots of time in the medical library long ago, so I keep a copy of the ancient Merck manual, as a hard copy for when the lights go out.