These are not dictionaries like The Canadian Oxford Dictionary (my desk dictionary). They are the kind of dictionaries that are fun to browse in as well as useful to look things up in; the kind of dictionary that can give you new ideas and solve problems far beyond word definition. In my opinion a dictionary such as these can be a far better writing prompt than any book of writing prompts. I have included books which are now out of print (for that reason not giving ISBNs for them) because I think that in almost all cases there is either a new edition or a similar dictionary available.
The Cassell Dictionary of English Idioms (no ISBN provided as this is an older edition). Works by listing words and then idioms associated with them. E.G. under cat among other entries: "like something the cat dragged in".
Medieval Wordbook by Madeleine Pelner Cosman (no ISBN as this is an older edition) - there are other such dictionaries around - I bought this one because it was cheap. Especially useful for writing historical stuff or just giving you ideas. E.G. under hair one may read that the length, colour, etc. of hair was believed to reveal personality, and more...
Oxford Dictionary of English Place-Names (9780192800749). An incredibly exciting etymological dictionary with a very useful introduction on how place names are derived.
A Dictionary of First Names by Patrick Hanks and Flavia Hodges. I think it is out of print but there are other similar name dictionaries around. Note this is not a "baby name book". It is an etymological dictionary of names which includes their variations and has an extremely interesting introduction. Please note: if you are interested in the actual meaning of names, a name dictionary is important. Stay away from "baby name books" as they almost always contain erroneous definitions (although they can have their uses, but why bother when you can have all this extra information as well?)
*Brewer's Dictionary of Phrase and Fable. My top browsing dictionary. You can find everything in here. It could spawn whole novels, honestly.It defies description, containing everything from myth, history, slang terms, expressions, etc... to definitions. If you are lucky you can find one used.
Dictionary of Symbolism by Hans Biedermann (9780452011182). Look up the number thirteen and find its Biblical and other literary significance and much else besides.
Oxford Dictionary of Slang. incredibly fun. I had to buy a new one because my children stole the first one to laugh at the names for body parts (as well they might; try it and see).
Oxford Reverse Dictionary. Very useful! Look up lament and it will give you keen, pibroch and elegyas well.