Looking for one book that walks you through the admissions process and how to maximize your chances of admission, especially if you're not in the top tier of students for your target schools? B+ Grades, A+ College Application: How to Present Your Strongest Self, Write a Standout Admissions Essay, and Get Into the Perfect School for You by Joie Jager-Hyman is an excellent place to start. With the exception of the soon-to-be outdated standardized testing information (read more about the changing SAT here), Jager-Hyman clearly outlines a generic college process from start to finish. Although it mentions "B+" students in the title and throughout the book, the message is really applicable to anyone who is running in the middle of the pack grades-wise for a college's typically accepted freshman class.
I, of course, will highly recommend that you consider also working with an Independent Educational Consultant who can personally tailor recommendations to your specific situation. But, if you're going to go it alone, at least get a resource, like this one, that clearly runs step-by-step through this process.