I love a good book to build a solid foundation for understanding, and understand that you can't rewrite the fundamentals every release, but does it matter that this book was published in 2012? I expect it doesn't cover all the highly relevant IoT features added in 4.2 and 5.0 and on...

The book doesn't cover things added after it was published, correct

Of course, I don't expect it to!

But in the past 6 years of your career, I expect you've studied those things a lot more than I have. Do you think that the book is still a good starting point for someone looking to progress beyond dumping confusing vendor binaries in off-the-shelf Bluetooth modules that seem to be energy hogs, to miniaturize and integrate custom firmware and onboard antennas on my PCBs?

yes, I highly recommend Heydon's book, even in 2023. The tech stack including l2cap, connection establishment, advertising, connection parameters, security, GAP, GATT, are all still relevant.

It's a good recommendation, but I'd prefer the whole thing were less complicated so, like USB, I could figure it out in a few days, finish my work and move on with my life.

I certainly appreciate one being able to make a career out of these things, but I'd rather not.

I would love to see a writeup that made bluetooth usable in a couple days. I suspect the aspects that make Bluetooth useful also make it too complicated.