The longest word in English is a topical one. Namely, if you were to inhale the microscopic volcanic dust spewed out by the Icelandic volcano Eyjafjallajökull, you might get pneumonoultramicroscopicosilicovulcanoconiosis. It translates roughly as 'a lung disease caused by the inhalation of very fine silica dust, causing inflammation in the lungs' (according to Wikipedia).
The word can be broken into seven parts which derive either from Latin or Greek roots. Pneumon(o)- means 'lung' (Gr.); ultra- 'outside, beyond; extreme' (Lat.); microscopic(o)- 'microscopical' (Gr.); silic(o)- 'stone' (Lat.); vulcan(o)- 'volcano' (Lat.); koni- 'dust' (Gr.); and -osis 'condition' (Gr.).
None of the works in our library are specialized enough to include medical terms this minute, but we do have a respectable collection of dictionaries. They can be found downstairs at REF 423, and upstairs on top of a small book case near the biographies.
Of course, we also have etymological dictionaries (REF 422 SKE and REF 422.03 CLA). Some intriguing special dictionaries include the Metaphors Dictionary (REF 423.1 SOM), The Dimwit's Dictionary (REF 423.1 FIS), and Words to Rhyme With, a rhyming dictionary (shelved at REF 423.1 ESP). My favorite, perhaps, is the Illustrated Reverse Dictionary (REF 423.1 ILL).