The IRS is a Bureau of the Department of the Treasury. This means that it is ultimately answerable to the Secretary of the Treasury.
The IRS publishes hundreds of guides to specific tax issues, along with forms, in print and electronic form. The print copies are available at IRS district offices, by mail request and as downloadable files in PDF format from the IRS web site. These are generally accessed by name and/or publication number.
Internal Revenue Manuals compile the operating procedures of the IRS.
Various IRS Forms & Publications are available here: IRS Forms, Instructions & Publications