Textbooks, professional literature and magazines, research material and reference works all have in common a high degree of meta-information. That is, there are footnotes, cross-references, lists of images, literature, authors, tables and formulae.
It's a super interesting category of publishing to work with and one we're very excited about.
It is also a type of work that lends itself to parallel publishing because they often come in small print runs and readers are interested in parts of the work. Why it makes sense to publish as interactive pdf / responsive HTML5 so you can get the full benefit of an interactive medium.