I have many, some old some new, my two most used are
Chambers Crossword Dictionary - which as well as serving as a sort of thesaurus, gives hints to what a word in a clue may indicate (anagram etc)
Chambers Dictionary itself is a bit of a Bible too, though pricey, shop around.
Bradfords Solvers list are a good inexpensive addition to the reference shelf.
It is a learning curve that never stops and charity shops are a great source for books on topics that may come up too.
Happy solving.