I want my users to set their previously read books in my app.
Is there any database with easily accessible API where I can get up-to-date book titles?
Open library has a goal of one page for every book, and has the data you seek.
It's run by the internet archive.
The British Library has a Linked Data API to the British National Bibliography (BNB): http://www.bl.uk/bibliographic/datafree.html
Examples of how to use the API can be found here: