So, which ones are necessary? In my house, I have a handful, on creating characters, grammar, plotting. One of my hardest-used is 
Complete Writer's Guide to Heroes and Heroines: Sixteen Master Archetypes by Tami Cowden, Caro LaFever, and Sue Viders (Lone Eagle). For really getting inside your characters' skins, this one is my favorite. It uses movie characters as examples for each archetype, with likely careers and backgrounds. When I'm having a hard time getting to know a new character, a quick flip through this one to the correct archetype is often all I need to put me on the right path.
In the same vein, your characters need names, right? 
20,001 Names For Baby: Revised and Updated by Carol McD. Wallace is great. I'm on my second copy and expect to need a third before much longer. I have other baby name books, but this one is pretty comprehensive.
After you've named your character and figured out what sort of person they are, they need a job, too. Careers for Your Characters: A Writer's Guide to 101 Professions from Architect to Zoologist is a good way to get a quick overview of all sorts of jobs your character might want to take on. I've discovered some interesting things in that book--I haven't had characters for them all yet, but I'll figure something out.
When you get to the actual writing, there's always some word you can't quite put your finger on, or one you realize you're overusing. 
Webster's New World Thesaurus by Charlton Laird is one I've also had to replace over the years. Again, I have others, but this is the one I use most.
You want to make sure your story doesn't drag, or that your readers don't figure out whodunit by the end of the second chapter, right? Then you need 
Conflict, Action and Suspense by William Noble. Conflict is important no matter what kind of romance you're writing. After all, if there's no conflict, why should a reader bother?
And just for fun, as a pick-you-up on those days when the rejections come in the mail (and they do, trust me), I love 
Pushcart's Complete Rotten Reviews and Rejections: A History of Insult, a Solace to Writers, edited by Bill Henderson and Andre Bernard. Some of the notes are horrid, and others will just make you laugh out loud--and be grateful for form letters.
I could go on. I have an entire small bookcase plus the shelf over my computer devoted to writing related books. But this is my absolute must-have list. If you're looking for ideas on something else, email me. I do have lots more.
Until next time, happy writing!
