How much research do you do on books before you decide to read them?

Personally, it seems that my interest in a book is mostly based on the context by which it became known to me. I'll find new books from other books, or from Youtube where the video references a book that's related to the subject at hand. But the problem is I'll make assumptions about the book, and read it from a skewed, biased angle, due to ignorance of the larger context from which the book was published.

Basically, I struggle to find the balance between knowing enough about a book to be interested in it, mindful of what I'm getting myself into, but without knowing so much that I spoil it for myself.

Is a book cover enough for most people? What gets you sold on a new book?