Funny that you say that. These last days I've been contemplating either:
* Getting a 9.6" or larger tablet for reading "free" technical ebooks (in PDF -- I can't stand the formatting of eBooks for technical stuff).
versus
* Buying used copies of the physical books themselves.
So, based on the recommendation of a HN reader, I bought an old Nook HD+ (9" screen with 250+ ppi) and I tried yesterday. The definition is OK for reading PDFs, but the interface of the PDF reading apps is atrocious. I'm giving the Nook to someone else and I'm spending $300+ on used copies of the technical books I want to read (mostly programming / reference manuals).
eBooks for technical reference are atrocious in several counts, first of all: typography (it's very common to find an orphan heading at the end of the page with the paragraph on the next page. This drives me nuts). Next, equations and diagrams. Terrible. Lastly, navigation.
To be satisfied, I estimate I need a 12"+ screen with around 300ppi. I think the iPad Pro can deliver this, but it's a rather expensive proposition.