[NOTE: I was sick yesterday, so I was unable to write a post.]
I have quite a personal library. I won't say it's enormous, but it's pretty big. A good number of my books are currently at my mother's house in Illinois (don't worry--I'm working on getting them back), and when they're finally at my house, I have to figure out how to organize them to make my library as effective as possible.
I have always organized my bookshelves alphabetically by author. However, my library has grown substantially, so I may need to organize my books a little more than simply by the authors' names to make sure I can find the books I need quickly and effectively.
For many writers (and readers) the organization of books is just as important as the books that are found on the shelves. And many people have very specific methods of shelf organization.
When I was younger, I organized books by size from largest to smallest (left to right). A friend of mine separates fiction genres and then alphabetizes. Another friend alphabetizes her books by author, and keeps books she's reading or using for reseach in a plastic bin by her desk. Another woman I know has one shelf for each genre. If she has a book that won't fit on the shelf, she has to choose one or more books to donate.
An easy way to keep your books organized is to alphabetize them by author (then by title, if you have books by the same author). This method mixes fiction with non-fiction with drama, etc., but since the authors are alphabetized, the books should still be easy to find. One note: if you choose this method, please use the authors' last names for alphabetizing.
Some people choose to separate fiction from non-fiction. They alphabetize the fiction by author, then organize the non-fiction books by subject. This can be particularly effective for research since you can go to the particular subject you need to find the books you need. Within the non-fiction sections, I recommend alphabetizing by author to make the books within each section easier to find. To separate the sections, some people I know use bookends to minimize empty space on the shelves. (Those of us with lots of books and little space know how important this is.)
Regardless of your preferences for book organization, it's important to organize your books in a way that makes them accessible, particularly for research. Books that are used frequently, for example, should be kept near your workspace. For writers, time spent writing is what makes money. As the dental cliche goes, "If you're not drilling, you're not billing." You don't want to have to spend time looking for a book when you've got a deadline looming. Find a method that works for you and stick with it.
How is your personal library organized? Why?