> It's still remarkably useful for a book that was published in 2013, but we are probably overdue for a "version 6.5".
That reminds me of a site I wish existed. It would mainly just present a table listing tools and technologies in column 1, and a year in column 2.
The year is the earliest year that books written for that tool or technology will still be mostly applicable to the current version.
For example, suppose you would like a book about Postfix (the SMTP server).
There are a few books on it, still available new. Hildebrandt and Koetter's "The Book of Postfix" (2005). Dent's "Postfix The Definitive Guide" (2003). Blum's "Postfix" (2001).
Does Postfix change slowly enough that some of those are still worth reading? That's what the site I want would tell you.
Of course it would be fine if the site had more detail. It might say things like for technology Foo books after 2016 should be fine, books from 2012 to 2016 should be fine except they will be missing major feature X and Y, books from 2008 to 2012 will be using an older, very different configuration system, and books older than that will be pretty much useless.