My own personal take is that if the summary is enough to grapple with and fairly evaluate, so that people who don't buy the book or get it for free can get still something out of that post (something comparable to your average Discussion post), then it's fine. Good material is good material wherever it comes from - Gary Drescher's Good and Real was not available for free but discussing it here with him was still fine because the material was very good.

(The last author to get bitten here was promoting a book that the LWer who read it described as extremely fluffy, uncomprehensive, and a good example of bad business books.)