That is still difficult to measure because the situation is evolving. The good old paper book is still a sure bet. However, there will definitely be an impact over the short or the long term.
Currently, that impact is felt mainly in the way people use excerpts. I am thinking of plagiarism, forms of satire or the commercial use of an excerpt used in an advertisement. We have seen all that. Technology does magnify those issues and makes it much more complicated to monitor the use of content.