Textbooks are expensive. So why would online textbooks be offered for free? It has to do with convenience and you see it all over on the internet. For example, Box.net offers you a few GB of free storage space, but if you want more security or more server space, you sign up for a paid account. Many other free web based services such as Surveymonkey, Flickr, NYT, Jing, etc. do the same, they hook you with their useful application and once you try it out and like it, you can opt to buy a more convenient (Pro) version.
So with free textbooks, it often works like this - you can read the text on your computer for free. You can also read some of the text and then decide that the convenience of being able to print it or buy the hard copy or get an eBook to put on your reader is worth the money and so you upgrade. The texts still tend to be cheaper because there is no upfront publishing cost.
Take this Writing for Success book on Flatworld, for example. Browse through all of the chapters on the left panel or buy the other versions. There is even a study guide. And you can choose between black & white or color printed textbooks, priced accordingly.
But not all textbooks offer the paid upgrade. Some just make you suffer through poor formatting or advertising. Bookboon.com, for example, offers free downloadable PDF files of textbooks. Most of the books focus on business, science and engineering, but here is one on Media and Cultural Studies. Jaques Lacan, Stuart Hall, Judith Butler, Laura Mulvey, Edward Said -- not a bad looking table of contents. The price? You get advertisements on every three pages of the "book". But hey, it's free Cultural Studies!
Other online publishers, like Project Gutenberg, are non-profits and only ask for a donation. Those, however, are more likely to have books for a literature class than textbooks.
Curriki has a list of 10 sources for free textbooks. Take a look.
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