If we do the maths on our iTunes U data throughout the figures are pretty daunting:
500mb worth of iBooks content (average per student in our eLearning introduction course) across 300 devices makes 150gb of content through our internet and piping over our wifi network. Scary, especially considering many New Zealand schools are still on adsl (we have a dual vdsl connection) and most of this content is downloaded by students in the space of a few days.
When you're dealing with that amount of content it really becomes worthwhile to get the iBooks that you've made up onto the iBooks store because OS X caching server will cache iTunes Store content, but not iTunes U content. You have to be more careful regarding the content of iBooks you submit to the store, such as copyright on the images used in the iBooks and you lose the ability to keep content in submitted books private. There's also the extra step of learners authenticating to 'buy' the free book (they don't need to do this when iTunes U hosts the content). But even with these caveats moving a couple hundred megabytes to cache-eligible content can improve overall network performance and, more importantly, the user experience lost while waiting to download.
Ideally I'd like to self host large iBooks Author books on our local webserver, but to date I haven't been able to get them to download and open on an iPad correctly.