How often should your books be updated

Not every day; as often as it takes to land the job. How often is not really the critical question; Is everything thats in my book consistent with my style, strong as individual images, and adding to the quality of the body of work as a whole? is a better question. Eliminate the weak images without mercy. Meanwhile, keep shooting, but beware nursing a limping project to death.
Bringing a project to a close is as important as starting one. Explore your idea deeply and thoroughly, finish it by making exhibition quality prints, hold a celebration exhibition, even if its in your living room, serve some drinks, and move onto the next thing. Its about the lifetime body of work, not any single image, no matter how lauded or how disposable. Once a body of work is finished, give it enough time to make the rounds and be seen so you can learn something from it and get some valuable feedback.