To become a Talent Manager....
First of all, you will need to start an internship (or working as an assistant) with either a Talent Agent, Talent Management, or a Casting Office. Mainly because...
1) You will need to establish connection with professional within the industry (names of casting director, agents, managers, working actors, and directors) and know how the acting business works (including contracts, working with a talent agent, submitting your clients via breakdown expresses and other services)
2) If you want to be a legitimate Talent Manger, then you need to join the National Conference of Personal Managers. http://www.ncopm.com
or Talent Managers Association Where you are required to provide three industry references (Place where you interned, and people that you worked with during your internship)
If you already have actors who are interested in having you as a manager, and if you don't have any 'connection' the industry people - you will not abe able to market your clients, you wouldn't know who to call or send your client's package to... So, the first thing for this career is really to KNOW the people, and establish a good working relationship with them, before you start taking clients.
For internship and assistant jobs, you might want to visit: http://www.entertainmentcareers.net
Here are some book suggestions that might help you get an idea on the "business" side of Acting/Entertainment Industry.
(some books are more about being an Agent - but really, Managers and Agent's function are very simliar)
"The Mailroom" by David Rensin (talks about breaking in the business)
"An Agent tells All" by Tony Martinez (The function of an Agent, and what he/she has to deal with)
"Hollywood Dealmaking" by Dina Appleton (This is a more technical book about contracts)
"Breaking into Acting for Dummies" by Larry Garrison (This is a quick, simple, and imformative book on Acting Business for Actors. You have to know what actors (need) do before you can managed them)
"How to Agent your Agent" by Nancy Rainford (This is a funny book where Nancy talked about how to balance relationship between her, her clients, and other people within the industry)