There’s a niche that doesn’t often get discussed: books about people. 

This can come in the form of short, unauthorized biographies like Clayton Geoffreys’s sport series, or Jake Brown’s In the Studio series. (I have bought books from both.) These books aren’t long, Caro-sized, biographies; they can be very short books, effectively long introductions to the subjects.

It can also look like Agate’s In Their Own Words series, or Larry Warsh’s ISMs series.

Zero to One started as Blake Masters taking notes on Peter Thiel’s lectures.

If you’re trying to write a book, this can be a great place to start; just start compiling information. 

For example, at three quotes a page, you only need 300 quotes to make a solid (small) 100-page book.

It’s a great way to practice ordering the information by importance or theme, promoting the book independently, and identifying good places to use imagery and art.