Good time travel book series?

In my many years of being a reader, I've come across a lot of books but very few time travel related ones. I'm a big fan of Doctor Who and am watching Enterprise for the first time from the beginning and I really am enjoying all the different concepts of time travel, not to mention I just watched Back to the Future for the first time. Are there any good books that entail perhaps a society that has something like exploratory teams that observe time as a means of research and/or fight other people who wish to alter time or other various dastardly things?