Bookboon is providing free PDF textbooks in a number of subjects. The subjects include Engineering, Mathematics, Science, and more. Bookboon states “Our books are legal and written exclusively for Bookboon. They are financed by a few in-book ads.”
When you download a PDF, you have to give your email and as stated above there are in-book ads. From Bookboon’s Facebook Info page,
The books are all written by university professors and exclusively written for Bookboon.com. The books are secondary litterature and is aimed at asisting the student when studying for exams. The books are around 100-120 pages and aim at explaining the all the relevant parts of curriculum in a short and precise manner.
IMPORTANT: Ventus Publishing and Bookboon.com does not share any information about individuals with any of our advertisers. The advertisers have also had zero influence on the content of the books, in fact the books are written before any advertisers are contacted.
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In addition to Mathematics sections, there is a separate section for Calculus with a number of PDF books.