However, if 90% of the planned use would be for reading ebooks while at a computer, would it be better to get a second monitor instead - I could then have the ebook open on the second screen and I would still have spare space.
If you own any ebook reader, please let me know your thoughts
If you're flipping through API documentation, or scanning tech books on Safari, second monitor is probably better.
I'd recommend an iPad over a black and white e reader as well, especially if you'll be reading books with diagrams or code samples, because the color screen comes in handy.
iPad - best device for everything other than just ebooks (news, games, multimedia etc.)
2nd monitor - best increase in productivity (YMMV). Put code, docs, chat, music, and more open at once. Your eyes move faster than Alt-tab.
Personally, I would go with a 2nd monitor since I don't read many ebooks.