While I am a great believer in digital books, please remember that the legal PDF copies of a bunch of coin books are limited to things before 1923 due to copyright laws. There are some worthwhile books on coins before that date but many of them are very dated, illustrated at best with line drawings and usually replaced by more recent and more scholarly works. The compilation disks are worth the $10 or so they sell for but they are more for curiosity than serious numismatic education.
I have been considering printing out one of my better page series (perhaps the Vocabulary?) at Costco so I could have a book to hold. That would cost something like $25 and several hours of formatting but produce a one of a kind collectible which would be worth exactly the same as the online version in terms of practical value ($0?). It seems silly but such is the hold books have on us of the pre-Kindle era.