Many "e-books" are nothing more than an on-screen PDF style version of the paper version, and in my opinion, are awful to use. My son complained about using them and I just dismissed him as whinging until I looked at a topic with him and realised how annoying and distracting it was.
In particular, they havn't been designed to be read on a single screen, whereas a text book is designed to have two pages open. For the book, you may have a diagram or photo on the left with description or narrative on the right - exactly how it was designed. Transfer that to a screen and you have to constantly flick forward and back from one screen to another.
That's just one problem. There are many others, such as with a paper version, you can copy a particular page and write on it, annotate it, etc., but you can't usually print a page from an e-book, i.e. no "print" or "copy" nor "save" options leaving you with the only alternative of the printscreen button which then means you have to resize, crop, etc.
At the end of the day, I believe it just makes things harder for the child, and anything that makes things harder isn't good when they need a rocket up their arse to do the work anyway!
I've bought hard copies of several text books which the school have only provided via e-books. My computer literate/savvy son, who spends most of his waking hours attached to his ipad will reach to the bookshelf for his homework rather than click onto the e-books - that says it all!