A reader sent in a link to this secret IRS 202-page report, 75 Years of IRS Criminal Investigation History, 1919-1994 (Document 7233) (Rev. 2-96) (Catalog Number 64601H), written by Donald K. Vogel, former IRS Assistant Commissioner for Criminal Investigation.  The cover of the report is stamped "Caution.  This document is for confidential use of authorized Internal Revenue Service officials only and not for publication." The report is available in both Adobe pdf and Microsoft Word versions.  The web site (The Memory Hole) claims that copies of the report obtained through the Freedom of Information Act contain multiple redactions, while their unredacted version was obtained in 1996:

Luckily for us, the IRS also published this report as an uncensored perfect-bound book and sent it – via the Government Printing Office – to Federal Depository Libraries in 1996. The IRS soon realized that the report shouldn’t have been released, and it made the GPO demand that libraries return it. Thus, some copies are still floating around. Above, you will find the unredacted version of the complete report/book, marking the first time that any version has been made available online. The Acrobat file is composed of scans of the book’s pages; the RTF file is text converted from the scans.

One source calls the report a "top-secret, authorized history that reveals the agency’s mistakes, investigations and (sometimes) arrogance."  TaxArticles.com says:

Why write a secret report? Former IRS historian Shelley Davis, who wrote Unbridled Power, Inside the Secret Culture of the IRS, is candid. "They wrote it to feel good," she tells Insight. "To pat themselves on the back and for no other purpose. They don’t toot their horn outside the agency. They just want to pat themselves on the back and look good."