The Times Literary Supplement - Known throughout the world as the TLS, it's authority is acknowledged world-wide and its approach is world-wide, too. The articles and reviews are on all subjects and often written by authors pre-eminent in their field.
The New York Review of Books - On the other side of the Atlantic, the NYRB does the same job as the TLS while concentrating more on American authors and issues.
Sydney Review of Books - Not to be outdone, the Sydney Review of Books takes care of all things book related on this side of the world.
Kirkus Reviews - A wonderfully comprehensive site reviewing all manner of fact & fiction. Founded in 1933, Kirkus has been an authoritative voice in book discovery for 80 years.