The “Person A discovered this book through Person B” feature is already super nice for building a community. However, it only applies when Person A looks at Person B’s profile and likes the appearance of the book (and/or Person A’s highlights and/or reviews). Ultimately, they’re still judging the book by its cover.
People who have already read the book and know Person B are actually more likely than Person B to know whether she’ll enjoy it. So being able to recommend specific books to specific people—which, if they accept the recommendation, would add the book to the recipients “Want to read” list—would help catalyse social discovery.