If, like me, you continue to be a Troff holdout, there’s good news from O’Reilly: They are now offering a free download of the original Unix Text Processing. The book has long been out of print and, I believe, the source code was lost. Now, however, a PDF file has been created, apparently with OCR, and the authors, Dale Dougherty and Tim O’Reilly, have generously made it available for free download.
Previously, the authors had found some PBM files for the book and released those. That gave rise to a project on the part of some of the Groff mailing list denizens to recreate the book from scratch. You can even get the source code for that effort, if you like.
This book is a real treasure and one that every Troff user wanted to own. I hunted for years before I found a used copy. Now you can just download the PDF.