Fabien Sanglard has made Game Engine Black Book: Wolfenstein 3D available for free download with updates to the DOOM edition. In the latest updates to the series, all reported errata have been fixed, both Wolfenstein 3D and DOOM versions of the books are available as 300dpi PDFs, and Sanglard has released the LateX source code of the books (Wolf3D, DOOM).
Readers are encouraged to gift to Sanglard for the books, but the high-res PDFs of both Game Engine Black Book: Wolfenstein 3D and Game Engine Black Book: DOOM are now available for download for free. Of course, hard copies are still available for purchase from multiple retailers.
Learn more and about all the updates from the blog post.