Book 5 in my series of Rediscovered Treasures is Boundaries: When to Say YES, When to Say NO, To Take Control of Your Life.
What this book Boundaries does is view proper protection of your time, space, and self as a crucial moral practice. The authors, Henry Cloud(gotta love a website called drcloud.com) and John Townsend, lay out their argument based on biblical quotes. So the thinking is Christian. But you don't have to be Christian to see that they're right: being a pushover is squandering life. If I let myself be run over and diverted from what I know I'm supposed to be doing, I'm up to no good.
Conquering Fears of Saying No
When I read this, quite a few years ago, I was entirely convinced. Before that I'd had a niggling underground conflict that a really good person would be able to do her stuff and everyone else's too. That conflict was dead well before I got to the last page. I've never even had to review the book again. It worked. Like a magic bullet.
Powerful book. Have a look at it, especially if you're in the habit of letting yourself be blown off course.