Here’s what I entered in my reply there, for people following the thread here:
If your ebook doesn’t share the same width to height ratio of the iPad (which is 4 to 3), then you’ll see that letterbox effect. This often happens because users are converting print books to fixed-layout epubs. The print books were designed to standard book sizes or custom ones, for that matter, but no one considered having to fill up the screen real estate for a mobile device.
To get the ebook to fill the screen, you’d need to check its settings in File > Document Setup in InDesign, and change its aspect ratio to 4:3 for the iPad if you want it to fill the screen when a single page is viewed in landscape mode (e.g. 8″ wide by 6″ tall). You’d need to half the width for facing pages (as shown in your screen shot) — 4″ wide by 6″ tall in this example — so the facing pages would be 8″ wide by 6″ tall.
If you’re designing an ebook from scratch in InDesign, then you’d start with the target aspect ratio for your page sizes from the get go.
AM