I was brought up cold recently when I decided to do an e-format game book. I was astonished at how much work was involved.
First, all the different tablets use different page sizes. So you have to do a layout that works for all of them, or do three or four different layouts. (There are lots of online tutorials for how to do this, using the various page layout apps like Adobe In-Design or Quark.)
Second, rulebooks require text and graphics to hang together closely (so that the section on "rolling to hit" is right next to the illustration showing how it's done
.) But that's very hard to do in e-text, because the text automatically scales and flows depending upon the zoom that the reader uses. So you have to "anchor" all graphics to certain bits of text (see problem #1 above for doing this in three different formats), and then make sure that when it flows that way, it doesn't get discombobulated from some other graphic.
Third, if you try to get around all of that by just doing a "Smart PDF" (i.e., an interactive PDF with links, etc.) then you still need to make sure that your smart features work in the different formats. I created some buttons that worked great when read on a computer screen, but didn't work at all when my wife tested them on her iPad.
In short, it's a pain in the ass. Doing layout for old fashioned books is actually a lot easier.
(And books don't require you to give 30% of your profits to the Apple Store, for the privilege of using their format!)