Lemon verbena is basically a relaxant, something it shares with many lemon-scented herbs.
I use this link a lot...

Here is a link that might be useful: herb site

The tea is used for bronchitis, flatulence, stomach cramps, nausea and palpitations, and as a gentle sedative.

As for 'herbalism', it depends whether you want to become a qualified practitioner or whether you just want to know a little for your own personal use of herbs as medicines.

I can't recommend a site for the first requirement, but there are many colleges around for you to look at. Also some correspondence schools (if they don't have a 'residential' component, forget it, because you need some hands-on instruction too). Try searching for 'herbalism courses' or similar.

As for the personal use, there are many, many sites out there you could learn much from. Start with the name of just one herb, search for 'medicinal uses X' and go from there. Or simply search for 'herb(al) remedies', or try by ailment 'herb remedies eczema'. And be prepared to sort the wheat from the chaff!

http://www.scs.leeds.ac.uk/pfaf/database/commonA.html

Here is a link that might be useful: some medicinal herbs