Fairly up to date, covers considerably more ground than most other FreeCAD books I've encountered, and the authors seem to know a lot about various CAD systems. There are also numerous nontrivial tutorial examples worked out in considerable detail.
As a downside, the book is somewhat pricy (very much so for the paper edition) and written in a very dry style, going through menu options one by one in a very formulaic language. I would recommend it, but it does not necessarily make for recreational reading.
But all of them are a bit outdated as for actual FreeCAD 0.20.x/0.21.x and RealThunder's FreeCADLink.
[0] https://wiki.freecad.org/Books