You might find that these maths textbooks are worth a look: http://bookboon.com/en/textbooks/mathematics
I downloaded the whole collection (single .ZIP file of 62.4Mb).
They look pretty good.
The reason they are free is presumably because they are funded by advertising, because each document seems to be interspersed with embedded and relevant adverts containing hyperlinks (URLs). Each time you click on an ad, it will send links to your browser. The first time you click on an ad, the window below pops up - note the default is already selected for you, so don't be too hasty. This is an Adobe version of "OpenCandy", I guess, but it's not as invasive.
Some of the ads look as though they could potentially be quite useful.