Most US reviews are protected under the First Amendment (free speech), and in Canada, freedom of speech is protected as a “fundamental freedom” by Section 2 of the Canadian Charter of Rights and Freedoms. There are of course countries where there is no or limited free speech, and any form of criticism will get you into trouble. A negative review can be seen as defamatory, in other words, libel.
“Libel is defined as a false and defamatory statement of fact about a living person or a business entity or product that causes harm to reputation. From the definition, it follows that: (1) you can’t libel the dead; and (2) truth is an absolute defense to a libel claim. (If it ain’t false, it ain’t libel.) It also follows from the definition that a statement cannot form the basis for a viable libel claim unless it can reasonably be interpreted as an assertion of a fact. That’s where the crucial distinction between fact and opinion comes into play.”
Basically, you are OK if 1) it is your pure opinion; 2) you review the book, not the author; 3) have no intention to defame; 4) avoid inflammatory words; 5) support your interpretation. (Source: Mark Fowler, Attorney at Law, in Can I (Successfully) Be Sued for […] Review of Your Book?, June 19, 2014, Rights of Writers blog, rtrvd. 2017-01-14)
If in doubt, don’t wait to be sued, get a lawyer to check it out.