In no particular order, here are the five:
Techniques of the Best Selling Writer
Writing the Breakout Novel
James Scott Bell
Plot and Structure
Scene and Structure
Empowering Charactions' Emotions
Granted this last one is not a book, but you can go to her website and order her lecture packet. I have the packet, plus I have had the honor sitting through one of her workshops as well as enrolling in her online courses. The content of her lectures are outstanding. I learned more about characterization through her workshop then I have reading books on the subject.
If you are in the market for buying books, add these to your shelf. If you are in a money crunch, then look at local library annual book sales, half.com, amazon.com to see if you can buy a used book. If you just want to borrow a book check them out at your library or borrow from a friend. You won't be sorry.