Here are some resources that may be useful to employers of employees that are undergoing a gender transition.
Sometimes the basic terminology is unfamiliar to employers.
The Wikipedia Trangender page has a good basic description of “transgender” and “transsexual”.
Human Rights Campaign
The Human Rights Campaign website has an Employer Resources page that provides various links to helpful resources for employers who are or will be employing a transgender person and wish to learn more. In particular:
World Professional Association for Transgender Health
The World Professional Association for Transgender Health (WPATH, formerly known as the Harry Benjamin International Gender Dysphoria Association, Inc. (HBIGDA)) is a professional organization devoted to the understanding and treatment of gender identity disorders.
WPATH Standards of Care is where you can find the “standards of care” for the treatment of gender identity disorder. (Don't get lured by the “Click here to order” near the top of the page; there is a also a free download farther down the same page.)
Center for Gender Sanity
The Center for Gender Sanity is a consulting service for transsexuals and their employers. Much of the content on the site is a little bit dated, but there is still some useful resources there:
The Center for Gender Sanity is the publisher of a couple of excellent books on transitioning in the workplace: Transsexual Workers: An Employer's Guide and Working with a Transsexual: A Guide for Coworkers.
The Diagram of Sex and Gender is a concise description of the four axes of sex and gender.
Transgender at Work Project
The Transgender at Work (TAW) project website has a number resources for addressing workplace issues for the transgender employees. In particular, they have a Resources for Employers page.