Hi- Ditto pp re PP's resources. (this is kind of late, I've never visited this board before).
Also, Contraceptive Technology by Robert Hatcher is sort of the bible of contraception for many people. A new edition just came out. He also writes a book called Managing Contraception, which is a pocket sized one. They are geared toward providers but written in very clear & easy to understand ways.
Also, check out the WHO's medical eligibility criteria document, it provides a lot of guidelines for different contraindications and use in various populations. http://www.who.int/reproductive-heal...lications/mec/
As far as STD's go, you can read a lot of info on the CDC's website as well as ASHA (american social health association).
Not sure if you're going to be dealing w/ colpo's and whatnot, but the ASCCP's website has all of those updated rec's. (again, provider driven but a good resource)
Hope you enjoy your volunteer work!