Most helpful reference book?

I find thesaurus's very useful/helpful/handy/beneficial/rewarding.

Whenever I am stuck for a word, trying to put something in a more interesting, or stop repeating the same word several times in a sentence, I head for the good old dinosaur book.

by Bryony Harrison

Probably the dictionary as I play Scrabble and sometimes just look through it to try to learn new words.

by Finy

WHo would have thought, all those years ago in English class, that the humble Thesaurus that I hardly understood how to use, would become my most referred too book? I also use my World History of Art reference book a lot.

by slang

Although our World Book Encyclopedia is 20 years old, it still serves as a fabulous reference book for our family. Certain facts, especially biographies of historical figures, are unlikely to have changed so it is still a valuable and reliable resource.

by Marlies@mbkedit

My Oxford English Dictionary, actually bought at OEP, when there for an 'Inspector Morse' Tour, & Exhibition at St Aldgate's Museum.
My Thesaurus. Ever helpful.

by donjo