I have worked as a UNIX system administrator for 25 years, with AIX, Solaris, and Linux (I do not like it). I've worked for one of the world's largest online retailers, the largest online stock trader, US DoD, among other companies, and am presently working for the world's largest transportation company.
But I don't consider myself an expert. To me, people like Adrian Cockroft, currently the Cloud architect at Amazon AWS and authored "Sun Performance and Tuning: SPARC & Solaris." Richard McDougall co-authored "Solaris Internals" while at Sun Microsystems and was CTO at VMWare and is currently in Cloud & Infrastructure Engineering at Google. Or "Solaris Internals" other co-author, Jim Mauro who is currently a performance engineer at NVIDIA. Another is Bryan Cantrill who was a former Sun Microsystems kernel developer and creator of DTrace, and is now the CTO at Joyent; who also contributes code for SmartOS periodically. Those are experts.
The last person I will mention whom I consider an expert is Brendan Gregg. He previously was a kernel developer at Sun Microsystems and was the primary author of "DTrace: Dynamic Tracing in Oracle Solaris, Mac OS X, and FreeBSD" and is the author of the book "Systems Performance: Enterprise and the Cloud." He also developed the ZFS L2ARC at Sun. After Sun he was the Lead Performance Engineer at Joyent. Presently he is Senior Performance Architect at Netflix.
Brendan Gregg has said he wanted to be an expert in performance so he picked up "Solaris Internals" (volume 1 because vol 2 wasn't even thought of at the time) and read it cover to cover. But this is the caveat. He stated when reading the book, he would not turn the page until he knew "everything" on the current page. It took him one year to finish reading it.
I only relayed that to show that it doesn't really matter if you read the XINU book, the Minix3 book, the FreeBSD internals book, one of the Solaris internals books. If you study it well and really learn the material, it will all be applicable to another operating system.
Having said that, I'd probably go with XINU if you also install it on the Beaglebone Black, Galileo, or Pi. That way you will learn all aspects of an operating system, and all code is in C. It is a lengthy book of somewhere around 900 pages. If you followed Brendan Gregg's advice to only turn the page when you know everything on the current page, you would be very knowledgeable.
After XINU, if you wanted to, you could read the others. "Three Easy Pieces" is really good.