On: The New ROM Antics – Building the ZX Spectrum 128

Same here! For those not similarly blessed, the Internet archive has a copy.

https://archive.org/details/CompleteSpectrumROMDisassemblyTh...

Of particular note, the use of a stack based VM for floating point.

I will presume that you know of the more recent tome on the ULA? "The ZX Spectrum Ula: How to Design a Microcomputer" by Chris Smith

Know of but don’t (yet) own. Strong recommendation?

I like it. It’s bottom-up, and I’d go as far as saying it’s the definitive book on the Speccy ULA.

http://www.zxdesign.info/book/

I’ll add it to my wish list!

Ditto. If you’re a Sinclair fan, all his books are worth a read.

http://www.ianlogan.co.uk/sinclair/books.htm