Dave's Garden is an excellent website with tons of information. A lot of it will just come from hands on work as well I think, nothing like seeing the plant in person and learning to ID it that way, able to see a lot more detail than most pics can offer. Also, if time permits, most any botanical garden will have opprotunites for volunteers, you will basically be slave labor, but you will learn a lot too, and the people in charge will/should be able to answer just about any question you have or point you in the right direction to get the answer.