I'm surprised that nobody has mentioned it yet, but the book that would be most practical to actually recover society after disaster from a purely technical standpoint would be the CRC Handbook of Chemistry and Physics.

http://www.amazon.com/Handbook-Chemistry-Physics-94th-Editio...

The sheer amount of information in that huge book is crazy.

Another alternative would be a wikipedia backup stored on an SD card and one of these or similar: http://www.amazon.com/WikiReader-PANREADER-Pocket-Wikipedia/...