https://forums.freebsd.org/threads/some-books-materials-for-getting-started-with-freebsd.90536/
https://forums.freebsd.org/threads/freebsd-development-books-papers-slides.1566/#post-9566
To begin with FreeBSD, the best BOOK is still Annelise Anderson's ...
FreeBSD: An Open-Source Operating System for Your Personal Computer
Second Edition, ISBN-10: 0971204519, ISBN-13: 9780971204515
This was released both in PDF and printed versions.
To go advanced, go for Michael W. Lucas' ...
Absolute FreeBSD: The Complete Guide to FreeBSD
Third Edition, ISBN-10: 1593278926, ISBN-13: 9781593278922
This was released in 2018 in print and multiple digital formats. Mind you, the 2nd Edition from 2007 is still also valuable!
For system development purposes, try Marshall Kirk McKusick, George V. Neville-Neil, Robert N.M. Watson
The Design and Implementation of the FreeBSD Operating System
Second Edition, ISBN-10: 0321968972, ISBN-13: 9780321968975
Source: https://forums.FreeBSD.org/threads/some-books-materials-for-getting-started-with-freebsd.90536/post-624448
