Other than a set of encyclopedias, a good book is Pocket Ref by Thomas J Glover. It has all sorts of useful information (conversion charts, formulas, reference tables, etc.) You can also look at Desk Ref

Another are any of the Ugly’s electrical reference books. Handy to keep with you if you do any electrical work or need to quickly reference some information. The Pocket Ref has pretty much the same stuff but Ugly’s is more focused and less broad.

And of course a dictionary and thesaurus are good to have too.