Any writing course, writing tutors and how-do writing books always stress the importance of a good dictionary. As far as I know, it seems like Oxford dictionary is the all-time favourites for most writers.
I use Thesaurus (the one that comes with Microsoft Word) more than my Oxford Shorter dictionary. But more than ever, I find myself trying to search for words from the "look" of a thing or locating a word from a concept. I have found below two dictionaries exceptional in cases like these.
Britannica Visual Dictionary is "a full color illustrated reference book of a wide variety of object from all aspect of life".
The Writer’s Digest Flip Dictionary is organized by using a reversed method for finding the right words and phrases describing people, places and things.
Example: I’m looking for the word for a female goat. Form Flip Dictionary, I find the word goat with this entry - goat, female: nanny goat. Also, the Flip Dictionary could double as thesaurus as well.
I highly recommend including these two to your collection of reference books.