There is no shame in asking about the ENTIRE compensation package. There may be shame in not asking. By accepting the job offer without knowing what your wages, benefits, vacation, work location, hours, company car... etc tells your prospective employer that you are not a very savvy negotiator. As an interviewee, you should always ask questions. Usually, benefits and compensation discussion is left towards the end of the interview, but you should always, always have this in your quiver of questions for your prospective employer. He will respect you for asking. Otherwise he will think, "Who is this dolt who doesn't care what the job pays?"
I like the term 'schedule of benefits package' Wesley Long used. Also, who will I work for, who will work for me? how large is the department, why did the previous incumbent leave, are good questions. They may not be willing to answer all questions, but ask anything that is pertinent.
In a tough job seeker market, you may have to take whatever offer you can get, but you still should know, up front, what you are getting into. Too, knowing the entire benefit package at the first interview, will help you make a decision should you be asked back for a second interview. You may not want to waste the time.
I'm not sure why the HR folks have you sign a letter that isn't really an acceptance of their unstated offer. Signing a letter that says you says you signed the letter is a bit strange. Read carefully, and tread lightly.