The Art of Computer Programming by Donald Knuth.
Roget’s Thesaurus
Bartlett’s Familiar Quotation
Black’s Law Dictionary
Peterson’s Field Guide to Birds
As long as it’s the International (now in 6th edition), which follows Roget’s original plan. The alphabetical thesaurii do more harm than good, IMO, since they don’t offer the same precision of the two-step Roget procedure.
And for cooking, i’d add:
The Oxford Companion to Food
Joy of Cooking (Rombauer’s 1975 edition only, not the committee-written abomination her greedy, lazy son recently presided over)
One other I love, although it’s rapidly becoming outdated since its 1995 publication, is
The Enclycopedia of New York City. Incomparable, enormous, fascinating.
Machinery’s Handbook
Don’t leave home without it. Or at least don’t leave for the machine shop or send a request to the machine shop without it.