Do you think I should just buy the e-book or buy a paperback copy of Gilead by Marilynne Robinson, as well?
This is a really good question, and it is probably a question readers and students will be asking more often now with the increased availability of e-books. It seems to me there are three things to consider before you answer this question for yourself.
The first consideration is what kind of reading you will be doing. There is a distinct difference, for example, between reading for pleasure and reading for a class assignment. Class assignments generally require more flexibility and ease in looking at different passages for quoting or for reference. In a classroom setting, the actual book would probably be much more useful and efficient to use unless the entire class is using e-books. Pleasure reading is another matter, as there is not the need to locate passages or reference the text with any regularity.
The second consideration is which format is most readable to you. Obviously this is a matter of personal preference, but think about when and where you are most likely to read. If your e-reader can be used effectively in any light and maintain a charge for the length of time you need, it is certainly a viable option for personal reading. An e-reader is perhaps more useful for those with vision trouble, as well.
The final consideration is cost. The e-reader is a significant one-time investment, but it can often be used to do other things, as well. Buying an e-book is cheaper than buying a new paper book, but it is much more expensive than buying a used book or checking one out from the library. As far as the environment is concerned, research indicates that perhaps the paper book is the best choice, but not by much.
Whatever your preference, there is a distinct difference between holding a book and holding a tablet in your hand.
There is a certain tactile value to “real” books, just feeling the paper, turning the pages.
Whether or not that is outweighed by the convenience of carrying a virtual library in a little tablet is up to you. Perhaps the compromise choice is to get the book at the library (virtually no expense) to see if it is a book you want to own before purchasing the e-book.