Modern books are too long, too. Let's take something like "Atomic Habits", which was god knows how often recommended in various places.. it can be summarised on a few pages max. (Gauss would probably only need 1/3rd of a page..) I think it's psychology and psychology only why those books are too long: 1. more paper seems like it's worth more, so they can charge the $20 or whatever .. 2. reading 4 pages seems trivial, reading 400 is an accomplishment. Having accomplished something (even if its B.S.) increases likelihood that we recommend it, because we want to brag