There aren't many games for win98 that I can run successfully on Win7.
That's true. Plus, the apps are platform specific, so if I switch to an Android or Windows 8 tablet down the road, I can't read the books I've purchased on the new devices. Paper books are only platform specific in terms of being only written in one language (actually a limitation many apps do not share). And it's probably pretty unlikely that at one point down the road you will stop speaking English.
With that said, though, it's not much of a limitation. They're a great value, generally priced between $1 and $4, and they're super coonvenient. Plus, they can be superior to their paper-based counterparts. The iPad remake of the classic Monster at the End of this Book is amazing, and far, far better than the dead-tree original could ever be. All the Dr. Seuss treatments (there are more than 20) improve upon the paper versions as well, though not as dramatically. Cheap, convenient (I'm writing this post on my iPad, and thus am currently holding in my hand my entire library of more than 100 children's books), high quality products made for a huge market of people who have disposable income (parents with iPads), are sure to sell well. 
I thought I should just chime in here.
While I think both of you have valid points, I think you're kind of missing the point that drventure is making. Or maybe I'm misunderstanding him and I've got my own point.... whatever.
Statistically speaking, What percentage of applications (or more importantly file formats) are still accessible from say... twenty years ago....
How about thirty years ago?
How much data is now permanently locked away on a medium that is inaccessible for one reason or another?
When you look beyond the computing age and look at the hundreds of storage mediums over the years, you begin to realize that none, not one of them, has lasted anywhere near as long as the printed word. And for good reason, That ---steaming pile of meadow muffin--- lasts damn near forever... as long as no one burns it... or leaves it in their compost pile... or lets the baby get a hold of it. Point is, there are manuscripts hundreds or even
thousands of years old still in existence. Can't really expect that of digital media.
Take a different media, such as audio. We've had wax cylinders, eight tracks, vinyl records, various cassette tapes, CD's and of course, digital audio. But how many can recall wire was used as a storage medium (someone did, it's been featured more than once on Eureka), dictabelts? How about those ridiculous DCC or MD formats? Yeah... You would be surprised to know how much audio (good stuff at that) is just locked away waiting for someone to at least convert it to a more modern format. (note: that used to be my job. Believe me, the number of people doing that and getting paid for it is tiny).
But TBH it's not as bad I make it out to be. Some team out there might put together an emulator to run these apps again so the expectation that such medium will become unusable as it has in the past has diminished.
All that said, let's move on.
Here's my suggestion. The books I tend to obtain first are either A) ones that catches my kids eyes. or B) ones that I remember from my own youth.
Some examples (off the top of my head) of books I want to share with my son from my youth are books like those by Richard Scary. For instance,
Best Word Book Ever have been altered at least three times (by my count) in varying degrees and is not the only RS book to be altered. When a book is adapted to a digital medium, I would like to see, wherever feasible, how the story itself has been altered. I'm usually not concerned with the extra, digital only, portions. More with how the story itself might have been altered to fit the medium. Of course, a couple of bucks isn't much, but my time is precious.
Just a suggestion. If you're only buying the digital books, it's not likely you would have the paper versions as well.