A monologue is a speech, usually taken from a play, that reveals something about the character. It is important to remember that monologues are said TO someone, and the actor should know his/her audience when his performing one. There a books published for actors that are just monologues. In order to best understand the monologue, the actor should find the original play to understand what the character's intentions are.  I am from NYC, and I have always found the employees at the Drama Book Shop to be especially helpful in finding good acting material. You can call to ask for book recommendations, song materials, monologues to fit your type, accent tapes, etc. and order over the web. You can also visit your local library and search for books with the librarian. If you are selecting a monologue for an audition remember to 1) pick a monologue that fits your type 2) pick an appropriate length monologue (usually 1-2 min) 3) find a monologue that you connect with 4) know who you are talking to.
For more tips, a Google search for monologue tips will give you pages and pages of advice!
 
Best of luck!

"My Last Duchess" by Robert Browning is an example of a Victorian writer who is popular for his usage of dramatic monologues.  This is one of my favorites.  Enjoy.
 
http://plays.about.com/od/monologues/a/mylastduchess.htm
 
Thanks,
Rachna

I assume that you are asking for audition purposes?  My daughter is a professional actor and monologues tend to be two minutes or less.  There are many books for teens and adults.  Some are actually titled "Two Minute Monologues".  You can find these in many book stores around the country or online.
 
One thing I do know about these monologues for auditions is not to "over act" but to "keep it real and sincere."
 
Good luck and if you wish to find out resources for actors feel free to add another question or contact me.