Building on Sandi's recent posting and my last one, here are four excellent books on writing fiction that I frequently recommend to novelists.
Writing the Breakout Novel and Writing the Breakout Novel Workbook by Donald Maass. Clear, practical discussions and exercises for novelists, with plenty of examples. After you've done as much work on your book as you think you can, pick up a copy of Writing the Breakout Novel Workbook and do the exercises from beginning to end. I'd bet money that you'll find a way to make your novel more compelling in any number of ways.
Plot & Structure by James Scott Bell. One of the best books I've ever come across on the mechanics of plot and structure and how they interact. Extremely user-friendly.
Gotham Writers' Workshop: Writing Fiction. Each chapter in this book has been written by a different Gotham Writers' Workshop faculty member and covers a particular element of the novel--plot, character, theme, point of view, and the like. What makes this book one of my favorites: Raymond Carver's short story "Cathedral" is included in its entirety in an appendix, and is referred to repeatedly throughout the book in order to demonstrate various concepts.