the trouble you have choosing is the same trouble people will have remembering, so regardless of your choice I would try to get both, and redirect the one you don't use to the one you do.

I usually go for singular since it feels cleaner to me. However, like @cacho said, you won't go wrong if you just get both and treat the one you like better as "primary".

depending on what you're going for, "good book" might also have a religious connotation to some people -- whereas plural might not

Look at other companies and compare.
- goodreads.com
- thriftbooks.com

They target a wide audience of people who love books, and the focus is on multiple books.

• Who is your site for?
• What's the focus?

I think I've failed with:
johns.tools
but you might want both with something like this.
my other domain is goodsign.io so goodsign(s).io doesn't really make sense here.