I just downloaded the free chapter. I don't see a Rails version number, but it does mention Ruby 2.6 and the book is dated thusly:
> This Version: April 2020 (Free-Chapters Edition)
> Rebuilding Rails is copyright Noah Gibbs, 2012-2020
> This Version: April 2020 (Free-Chapters Edition)
> Rebuilding Rails is copyright Noah Gibbs, 2012-2020
Have you got any other recommendations for intermediate to advanced rails topics?