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, pictorial,
literature-, author-lists, tables and formulas.
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 publication because it often comes in small print runs where readers are often only 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.