You don't need more than three writing books. I have more but wish I didn't. They're not bad, they go over the basics, but if you want to be a writer and you've already started then the elements for writing have already been picked up from your favourite books/films/poems.
Focusing on prose, I give you the three best writer's tools available in book form:
1) The Three A.M. Epiphany
2) The Write Brain Workbook
3) Lights! Camera! Fiction!
The first one, I have barely worked my way through a tenth of the exercises, but they are so good, so versatile, that for each exercise I find myself writing up to ten really different storys or fragments. That's all that needs to be said about it really. The perfect exercise book.
The second functions as a warm up for a day of writing, sometimes invaluable when you can't bring yourself to continue what you started yesterday and you want something fresh to do to take your mind off of your serious work. I don't think I've created a single piece that I would call professional from this book, but that's not the point. It makes writing more enjoyable and hence made me far more productive in other endeavours.
The third may fluctuate between the obvious and the inspired but whether you find the book a revelation or a waste of time, the worksheets in the back are really the most useful tool I've ever had for writing a novel. I don't feel obliged to answer every question but it makes me look harder at my character's backgrounds ('How does this character see himself in regards to his occupation?') and plot.
If there are any books that you also think are invaluable I'd love to hear your reasons, but what I've found so far is that most other books have failed to provide me with anything that writing practice won't provide.
(And yet I'm still reading them... XD)