I took time off between my Bachelor's degree and applying to grad school, and I used that time to explore a) if I really truly wanted or needed to go, and b) what the heck I might want to study. This post is intended to help people with the first part more than the second. I read a lot about what to expect (or not expect) from grad school.
I'll start with the resources that are directed at graduate school in science as a whole.
A broadly applicable (for Ph.D. programs) piece of advice was written by Phil Agre, Advice for Undergraduates Considering Grad School. It walks you through generally what grad school is, research, and the application process. This guy seems to have written several interesting pieces of advice for a variety of fields.
Physicist Steven Weinburg wrote a piece published in Nature called Four Golden Lessons, and it can be found here with a response from John A. Duley.
Then there are books. In my first year of grad school I had to take an ethics class that required us to buy Getting What You Came For: The Smart Student's Guide to Earning a M.A. or Ph.D. I was initially put off by the title, but I read most of this book and will keep it to refer back to occasionally. It gives a great overview of things to consider when deciding to apply, how to decide between Masters/Ph.D. and their differences, and financial issues. The introduction is admittedly pretty bleak, but you should read it. It will discourage you from going to grad school because you feel like the extra letters alone will get you a job (it won't) or because you don't know what else to do with your life.
I also read The Ph.D. Process: A Student's Guide to Graduate School in the Sciences before I decided to apply to know what to expect during the different stages of grad school. This book focuses only on Ph.D. programs unlike the first book which also addresses masters programs. There are quotes from grad students in different fields throughout the book. People who are unsure what grad student life is like may especially want to read the Life of a Graduate Student chapter.
I recommend checking out either or both of these books from a library if you're not sure at all what to expect from grad school and just beginning to explore your options. If you find them useful and you decide to go to grad school, try to get yourself a used copy.
Next there are resources aimed at biology students, such as On Graduate Studies in Biology by Kurt Reinhart (pdf). Page 6 has an especially useful "checklist for picking the right advisor" which has a list of questions you may want to ask. I have this file highlighted yellow in the Finder to remind me to revisit it occasionally, as it has advice applicable to the duration of graduate study.
The next two pieces should be read together as they both came out of the graduate program in Zoology at Berkeley. They are relatively general in their advice and would probably be of interest to people who don't have ecology/evolution/behavior aspirations. First read Some Modest Advice for Graduate Students by Stephen Sterns, then read Reply to Sterns (both available as pdfs here).
Finally, there is a slim little volume called How to Do Ecology that is, as the title suggests, aimed at a specific audience. An even better title for it would be How to Become an Ecologist, since it talks you through the grad school process. If your idea of being an ecologist involves spending hours, days, or weeks on end alone with the wilderness, read the chapters on Working with Other People and Communicating What You Find to see how most ecologists have to deal with the social/communication aspects of their career. The three-page conclusion summarizes the most important unwritten rules of ecology as elucidated by the authors.
What resources would you recommend? If there's interest, I might write longer reviews of the books. Happy searching!