In almost 100% of the cases, the answer is "yes".

However, depending on the license of the books, you will probably need to indicate the original author of the book, link to the original book location, and add a note that it was released under the XX license (this is true for every CC-licensed book).

The biggest resource of open source books is, I would say, GitBook: https://www.gitbook.com/explore

Now, since this is a startup, I would be careful about non-commercial CC-licensed books (they have NC in the name of the license). If that's the case, you are kind of making money out of distributing them, so I would be careful about including them and discuss about including them with a lawyer. If they don't have NC in the name of the license, you're good to go.