Structurally, the most challenging book I've ever read was "The Message of THE QUR ĀN" by Muhammad Asad.

Start with the fact that the QUR ĀN itself is extremely non-linear. So much so that I think that this alone requires a great deal of study to address.

The text is 2 columns, the original Arabic adjacent to his English translation. There are copious and often long footnotes. The footnotes cross reference other footnotes, sometimes in chains. I read only the English.

I had to read it 4 times. Once just ignoring footnotes. Again, this time including just first-level footnotes. Again, following footnote chains back to their sources in the text. Finally, to reread just the text after pretending that I had everything figured out.

It took me a year to get through it to my satisfaction, although it was not the only reading, or even major project.