Strangely enough, the file extension was AZW, and not AZW1 or TPZ.
The book contains mostly text, aside from a few diagrams, and the front matter seems to be mostly scanned (copyright page, for example).
Chapter headings seem to be SVGs, and the first letter of a sentence in the beginning of a chapter is a fancy glyph.
So far, my attempts at converting this to ePub/Mobi have failed. The inline table of contents ends up looking different than the original book:
Original:
EPUB:
And that glyph that starts a sentence is gone:
Original:
EPUB:
