One of my best book purchases ever is one called Plant Identification Terminology: An Illustrated Glossary by James Harris and Melinda Harris.
First is the glossary, in which every botanical term is defined and - here's the really good part - illustrated with a small line drawing!
Then there are sections on terminology by category - stems, flowers, leaves, fruits, etc. - again, all illustrated.
It was not terribly expensive (about $14 if I recall correctly) and is excellent.
Having a glossary like this is really a must if you really want to get to the point of keying out plants.
You may be able to find a good book of the flora for your area (e.g. for my area, Flora of Alberta) - these will usually include a glossary section, not normally illustrated, unfortunately.
The Harris illustrated glossary is a huge help and I highly recommend it.
