Bookboon.com ; free academic e-books

Bookboon are an outfit based in London who, well you might have worked it out from the title, but in case you didn't, produce free academic textbooks in pdf format. They are all written by academics, so you'd expect them to be trustworthy.  Simply register an email address for their newsletter, tell them whether you work or study  and what you study or work at and then download the pdf.  What's the catch you ask.  Well, the only catch if you want to call it that, is that advertisements punctuate the texts.  To be fair, they're not that intrusive, as they generally appear at the end of chapters and are not too garish and the books are written by specialists in their fields, so why not take a look