Best C++ WinAPI Book Since Petzold?

[Igor Vartanov]

and hundreds of new functions added to the Windows API, since it was written.

You have to distinguish GUI programming (what Petzold's book is completely about) and programming client side apps making use of kernel services (what Richter's book is completely about). All the hundreds of new functions you mentioned represent evolution of kernel services most of all, but not GUI.

I did find "Windows via C/C++" by Richter, written in 2007, but it seems to be targeted at people who are already comfortable with the API and are just looking to get the last 10% out of their code.

This is where you're wrong. The book is focused on kernel services which make an API subset completely different from GUI oriented API. And I assure you, the book definitely is for beginners.

And I would warn you about estimating it as "the last 10%", as the estimate better fits with Petzold's book. I just cannot imagine a book that would cover every single part and pit of numerous Windows APIs.