It's time for a blunt service announcement
When I see an e-mail that starts with "Hey, I wanted to know if one of you would like to read my book", I am about 90% likely to just close the e-mail and walk away. Maybe it sounds cruel, but from my standpoint, why should I read a 300 page novel of yours to give you feedback when you can't take the time to e-mail me personally? And agents, you should know better! If you're representing an author, don't throw them under the bus by sending out a random press release to all active bloggers!
I've mentioned it in my review request tips before, but a reviewer is doing a lot for you as an author. I know sometimes people think we're getting a free book, we're more than compensated, but the internet is honestly full of free books. And I still buy plenty of books on the side. My reading list isn't shrinking anytime soon. And while a book may take 3 hours to read, a review can take another hour to write. That's time I'm not getting paid for. But I feel feedback is useful, so I set out to give as much as I can. Most of us review because we care about authors and we want to help. But starting out the relationship with a casual mass e-mail is a big no no.
Are you going to send an e-mail to 30 colleges and say "Hey, your schools look cool, message me back if you think I'd be a good candidate?" Chances are, no. I can get 10 review requests in a single day. I'm not going to take them all, and chances are, if you bulk e-mail me, it's going to make that elimination process just a little bit easier.
Send one e-mail at a time people. That is all.