EBooks (the software ones) are a complete failure when it comes to reading literature or ny other sort of non-technical documents , BUT , EBooks(specially in CHM format) r very good to keep running as reference material when u're programming or doing something technical because they r easily searchable so it becomes much easier to locate the erlevant material in it .
for example in my case , when i program for ny framework , say PHP , then i keep the php manual running and as the Manual is in CHM and completely indexed i just type the function name n i get it's whole usage , description n examples , simple isn't it