1. The National Book Critics Circle conducted a survey among its members about ethics in book reviewing, and I'd recommend that all bloggers who review books take a look at it. Agree or disagree, we bloggers need to be aware of what the ethical issues are. Here is one example from the post at the NBCC's blog, Critical Mass. (The full survey is linked there, as well as an earlier one from 1987.)
... new Net-based aspects of our literary life appear not to have settled in enough to create clear-cut ethical judgments. One example is the split that resulted from Question 17: "Should a literary blogger review the book of another literary blogger to whose blog she or he links?"
33.4 said "Yes."
23.4 said "No."
22.5 percent were "Not Sure."
20.7 percent retreated to "Other."
2. In "Critical Condition" at The New Republic, James Wolcott devotes many words to Gail Pool's Faint Praise: The Plight of Book Reviewing in America and offers his own advice, in the form of recommended reading, to novice reviewers.