No way. I love my eReader for reading novels and other books that I plan to read "cover to cover," but I wouldn't want textbooks in that format. (And I personally can't imagine reading even a novel on a smartphone.)
While I may sometimes read from a textbook chronologically, I largely find myself reading about things as they come up (in my apprenticeship, or my pregnancy, or I hear about something on the news or in the community that sparks my interest). So, for example, we had a baby born a while back with an extra finger on each hand. I hadn't come across anything about that in my reading, but when it came up it made for a really interesting and pertinent time to go looking for information about that and to do some reading about it. So I do a lot of browsing and perusing and index-searching on topics that interest me at a given moment, and I think that would be difficult to do on an eReader.
And, like Starrlamia pointed out, charts, graphs, illustrations and photos would lose something on an eReader (and be totally lost on a smartphone.
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