If you can't think of anything nice to say...

...your mother and/or your conscience would tell you not to say anything at all. Then again, you might try one of these pithy (snarky?) quotes in your next difficult review:

I have read your book and much like it.

Thank you for sending me a copy of your book. I'll waste no time reading it.

This book fills a much-needed gap.

These quotes are all by Moses Hadas. They and others can be found at The Quotations Page.