Electronic books are a substantial portion of the book market and likely to grow larger. So far they have been concentrated in the genre fiction markets where relatively few books have maps. But e-book editions are increasingly becoming the norm in general non-fiction and history markets where their print counterparts often do have maps.
The e-book presents a challenge for map-makers because of the limitations both of the format and devices. Amazon's Kindle is very much the largest-selling format and it will not accommodate files larger than 127 kilobytes. Moreover most of the devices on which e-books are read have monochrome screens of modest dimensions. The most common devices have screens with resolutions of 167 dpi with quite limited dynamic range.
In drawing maps for my own e-books I've taken my lead from the maps in early-20th century newspapers, which had some broadly comparable limitations of medium. As this suggests, quite a lot actually can be done within the e-book compass.
Nevertheless, there's no doubt that the limitations are severe in terms both of quantity of information conveyed and aesthetics. And one sees a lot of very bad maps in e-books as a result of failure to recognize and accommodate to the limitations.
As a workaround, for my latest project I've made up a PDF booklet of A/A4-sized maps that the reader may download, allowing much greater freedom for map-making. Additionally, this helps the reader avoid the old problem of needing to flip between text and map, a particularly cumbersome maneuver while reading an e-book.