Would anyone wanna become a beta tester for a book-based social app I'm making?

Most people aren't going to want to sign-up without knowing more. The intro sentence doesn't do enough. You need to demonstrate the purpose of the product and the value proposition to the customer (i.e. your users).

No one is going to contribute their e-mail and their time for free. No one does anything for free, really.

I have no idea what I'm getting into by signing up. Make it more clear and I'd be tempted!

This is probably better as a "Show HN". Just FYI.

I agree with the other comment. You aren't showing any value. And honestly, this looks like a scam to get email addressed (the fact that you are posting it here and asking for feedback means you might not be scamming, but looks matter).

Can you afford a domain name? Go get one. Seriously. You are asking us to invest time and effort into your idea and yet you haven't invested in a $5 domain name? Go get a domain name.

Take 4 hours today to go spruce up the landing page. Cheat and look at other landing pages and add some general elements from those. Show us what you want to do. Give us one good reason to sign up.

By the way, I think the idea is interesting enough for me to write this note :) so you might be on the right track!

>By signing up, you are agreeing to our mumble jumble of ding dong.

Needs more effort, or to be removed.

>This website (in beta) lets you enter the books you've read and the genres you're interested in, and matches you with people with similar taste based on it.

Removing all the "this x lets you do y" may be helpful.

Maybe "Connect with your bookmates" or "Swipe right for book readers". It's connecting you to the friends you didn't know you had.