My guess is that this is related to spatial memory. A lot of memorizing techniques somehow use space to anchor memories on; eg the old Greeks used to walk down the city’s main street while rehearsing speeches, and anchoring topics spatially onto the buildings.
A textbook is not exactly a street you are walking down, but it does have a certain physical place for every topic, and our brain can anchor it there, like „let me see, differential equations are in the last third.. ah there is this other topic, I know that diff equations come right after that“.
You've conveyed the exact problem I have with ebooks for any kind of technical reference. I read novels in ebook formats and have no problem with there, but after trying several O'Reilly and other publishers works I just found somehow unable to take in the information I was reading. Quickly scanning through to look something up is just impossible and built-in search functions are just miserable.
This is a major problem for me when reading digital material, especially if not on something like a PC. On the comuter I can at least Ctrl+F for the part I'm after.
I have noticed a similar problem. Our company started to offer information in "wiki" format instead of traditional manuals, but I still prefer manuals and get confused quite quickly on the wiki. I think it's because the hyperlinked nature of the wiki makes it harder to remember where things are.
A textbook is not exactly a street you are walking down, but it does have a certain physical place for every topic, and our brain can anchor it there, like „let me see, differential equations are in the last third.. ah there is this other topic, I know that diff equations come right after that“.