Ah, that makes sense. Referencing multiple ebooks away from a computer would be hugely inconvenient.
For me, using a book for reference almost always means I'm writing code and have the online Kindle app open in one or more browser tabs. I didn't even consider the use case where somebody would be referencing books away from the computer.
Even while on the computer its a surface area thing, which can be partially fixed by multiple monitors, but only partially.
So, I'd like to simultaneously see a PDF of the circuit board layout, the schematic diagram, the bill of materials, maybe some text commentary (best install C21 first, as U14 and R75 will be in the way later...) and maybe a datasheet of some critical ckt component. And if I'm doing something vaguely computer or microcontroller related I'm going to want another machine just for that. So 4 to 6 screens is reasonable. There are quad output video cards, another solution is I have three monitors on my desk both at work and at home with multiple computers connected to them.
For me, using a book for reference almost always means I'm writing code and have the online Kindle app open in one or more browser tabs. I didn't even consider the use case where somebody would be referencing books away from the computer.