I have an InDesign Book (with two sections) that I need to turn into a single "Publish Online" document. The only way I could achieve this was by creating a separate document and duplicating everything into a single document. Kind of defeats the purpose of using "books" if I have to manually merge the documents before using Publish Online.
The thing to keep in mind is that the feature Publish Online was only first introduced in June 2015. It's already had two major updates since then. I think you'll need to be a bit more impatient because this not the only feature the engineers are working on.
The truth is that most people create single documents, not books, so it's not as high a priority.
Publishing Books online (indb) is a huge priority at my company. we produce 25 page magazines with up to 8 contributors, meaning at least 8 separate sections (indd) files. This is called "group publishing" and is one of the main reasons we use ID. Without being able to publish Book files online, the CC upgrade is useless. I am evaluating to recommend the upgrade to our team. Without this feature, we've no reason to move from CS6 to CC. Serious misstep and false assumption about most people producing single docs.