PDFs
I know it was implied to be physical book, I usually read academic stuff, and having ability to select stuff and searching, dark theming, and ability to carry my older almost closet full of books, in something like 100-200 MBs of PDFs is just great. There are times when I actually had physical books, I would scan and ocr if i could not find a digital copy from the 7 seas.
When I actually would get stuff, Hardcover (preferably jacketted) because they were thick (like 400-1400 pages thick) and not having hardcover meant the covers would have a half life of something like 50 uses, maybe less, or atleast it would get stained.