Case for E-Books on Programming Languages
I am a big fan of books, blogs, screencasts and articles which are related to computer programming and software design. I spend almost 90% of my productive time with computers that are connected to internet.
I have noticed that buying e-books is the right way for me. This can be attributed to my way of living on the edge and the volatile nature of the latest programming languages. E-books provide an easy way to upgrade the book and not to mention greatly reduce the usage of paper (so eco friendly 
). I know purists will hate to give up the paper book but I am not a purist.
I have recently bought couple of rails books, pickaxe (Ruby) book and Erlang book from The Pragmatic Programmers and have been very satisfied. I get regular email alerts whenever there is an update or new version of the above books and its very easy to get hold of the new copy for no extra charge. However, I am not sure whats the deal with other e-book sellers.
Other than the above there are plenty of other uses like, hyperlinked resources/references which are just a click away from the book, one can copy/paste the sample code instead of laboriously typing the same stuff, no need to context switch between computer and book, no need to switch between keyboard/mouse and pen/pencil, easy to take notes, easy to share snippets of books with friends and many more.
E-books are also advantageous to authors as they can bring the social networking aspect in editing of book. This is suitable in computer programming related books which would require lot of useful code snippets and examples. There are many people who want to contribute for this cause but are not ready to write a book! One of the recent experiences, where e-book might be the only way to go was Rails Recipes by Chad Fowler, this book as the title says is destined to constantly evolve and its not desirable for the reader to buy a copy of this book with each new recipe added or get a seperate addendum for each revision. With e-books its just a click away.
I am sure there are many out there who would want to adopt e-books. So a clear indication of when to buy an e-book (I would say always but …) is if you know or feel that a book will have more revisions in near future.