I really like Nelson Coon's The Complete Book of Violets, which is out of print but can often be found online via out of print stores.

Pansies, Violas and Violettas by Rodney Fuller is excellent and out of print as well, but worth tracking down.

Roy Coombs new edition of Violets, The History and Cultivation of Scented Violets is excellent.

Try Abe Books, Alibris or Powells Books for out of print bookshops online.

All three of the following web sites are filled with excellent articles, photographs, etc:

http://myweb.tiscali.co.uk/heartsease/nvps/

www.americanvioletsoceity.org

www.sweetviolets.net

Have Fun!
vicki