I've read many books on plotting and structure, but this is probably the best so far. It doesn't offer a miracle cure for ailing plots, or promote a magical formula for coming up with the perfect novel, or divulge any flashy techniques - instead, it provides a nuts-and-bolts, back-to-basics guide to developing a strong foundation for your novel.
It has to be said that a lot of books on writing can come across as quite patronising - not so here. The book has a very friendly tone and puts its points across in a simple, straightforward manner that's always easy to read and often quite entertaining. There are many tips and exercises, all of which are practical and useful.
I read this before embarking on my current project, but I imagine it would be useful whatever state your novel's in, as many of the hints and tips can (and should be) applied throughout the whole novel-writing process. My copy is already covered in highlighter pen and scribbled notes in the margins, and I have no doubt that I'll be returning to it again and again. I see this book as an essential reference guide to the craft of plotting, and would encourage anyone who's serious about novel writing to invest in this. If you only buy one book on plotting and structure, I'd make it this one.