We do sometimes get concerned teachers asking whether or not "publishing" books is safe and secure. There's something about the terminology that puts some teachers off.
Of course, we use the term publishing because we're talking about books, and it's a nice fit - "publishing a book". But what we really mean by publishing is simply sharing a book online - it's actually the same thing.
When you publish a book in Book Creator, you get a secure link to that book. No-one can find it by searching online unless you choose to make it public. It's the equivalent of sharing a Google Doc with the access set to 'restricted to anyone with the link'. Only those people you choose to share the link with will be able to view the book online.
Rest assured, publishing your book online is completely safe.