Apple does have a valid trademark for the term "IBOOK" relating to computers, and the company once sold a computer called the "iBook". But as the lawsuit points out, Apple never used the term for electronic books until the Apple iPad was launched in April 2010. The suit says that Apple's use of the term on the Apple iPhone and iPad will "overwhelm the good will of plaintiffs’ ‘ibooks’ and ‘ipicturebooks’ marks and render them virtually worthless."
source: Bloomberg via BGR