I also forgot to add, IRS publications, especially Pub 17
http://www.irs.gov/pub/irs-pdf/p17.pdf
are great references if you are unclear about anything in the tax materials.  But don't worry, you don't have to memorize everything in the pubs, these are just good references.

If you are not already very familiar with the 1040, 1065, 1120 and 1120S forms, it is also very useful to familiarize yourself with the forms.  Remembering how the form is laid out can help you answer questions asking for such details as what types of income/expenses are partnership level versus separately stated to the partners, what types of credits are refundable vs nonrefundable, whether an deduction is above-the-line in arriving at AGI vs itemized deduction, etc.

All IRS publications
http://www.irs.gov/publications/

Tax forms
http://www.irs.gov/app/picklist/list/formsInstructions.html