Library reference rooms include a wide variety of dictionaries, ranging from standard desktop issues like Merriam-Webster's to those designed specifically for single interest topics like foreign languages, abbreviations, slang or regional dialects.
You will also find academic and professional dictionaries like Black's Law Dictionary, Stedman's Medical Dictionary and the Oxford Dictionary of Natural History. The entries in these specialized volumes contain the terminology used in specific fields.
Unabridged dictionaries, in which you can find the most obscure words, learn their definitions and pronunciations are standard reference books in most libraries as well. The largest, in any language, is the Oxford English Dictionary (OED), now in its second edition.
The OED consists of twenty volumes and, in larger libraries, is often available in an electronic format. It not only defines each word but tells its etymology--or history--as well. It details the various meanings a word has held in the past and provides examples of its usage from the earliest appearance in the language to the present.