Personally, I think e-books will be practical when these requirements are met:

1. The reader isn't bulky.
2. Random access is easy.

#1 is probably doable within the next few years. LCD screens are being made from modified ink jet printers, so we can expect readers to become easy to fold up, stuff in your pocket, etc. Things we can't do with laptops, tablet computers, and PDAs (PDAs come close, but don't go quite far enough).

#2 might be asking the impossible, but I still hold hope that someone will do something clever. By random access, I mean that I can open a book to the general area I know a subject is at, having read over the book before and need to look up the specifics. Hyperlinks are a good attempt at this, but aren't quite enough.

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated