As others have mentioned, man pages, the Handbook and also the FreeBSD FAQ are good starting points. I'd also recommend Absolute FreeBSD: https://www.amazon.com/Absolute-FreeBSD-3rd-Complete-Guide/dp/1593278926
As an experienced Linux user many things will seem familiar, so I guess you could read that book only topically
I’m currently reading through this book. I really like the authors writing style and his sense of humor.
https://www.amazon.com/Absolute-FreeBSD-3rd-Complete-Guide/dp/1593278926/ref=nodl_