This is just a note on a web source I have just come across that has freely available digital copies of some important books, the Universal Library.
The 'book of the month' I would choose from this source is Eric Williams's Capitalism and Slavery, first published in 1944 and quite difficult to get from bookshops. It is a classic text that explains both the mechanism of slavery for capitalism through the example of the Caribbean slave trade, and the context for the moves to abolish slavery (first the slave trade, then, much later, slavery itself) in the British Empire in the 19th century.
I referred to this text in my book review of The Sugar Barons, 4 August 2011 on this blog, and am pleased now to have found it on the web (I wasn't looking before, since I had the book!).
Here is the link: