I am not really sure if these are considered apps. I usually use these websites to get book recommendations.

* Hacker News Books - https://hackernewsbooks.com, the weekly most mentioned books on HN

* reddit reads - https://www.redditreads.com, the books that Reddit users are reading

* Shepherd - https://shepherd.com, the book recommendations from authors, creators, and experts.

I found interesting books from blogs sometimes.