I've raised awareness of this blog over at CPCwiki, but it's well worth checking out Ken Shiriff's blog.
As the Z80 is a very close relative, I'd say it's well worth keeping an eye out on this very well written blog. The technical discussion of the 8085 undocumented flags is outstanding - he reverse engineers the flags from looking at a photo of the silicon, translates that into gates, works out why the existing documentation is wrong and what the flags can be used for.
It's worth remembering that the 8085 is somewhat different to the Z80, however. It's still a fascinating chip to look at and by looking at the differences between the two and how they diverged from a common ancestor is very revealing.