Split an existing document with primary text frames into book files.

So I have a document which is 90 or so pages and I realised I should have created a book and split it into 3 separate files but didn't know how long it was going to be when I started.

I used Primary Text Frames so if I duplicate the file 3 times and try to remove the required pages to bring it down to 30ish each (although I'm open to the optimum number of pages, can't seem to find a definitive answer, I guess it depends on the content and how big the files are?), all the pages obviously just auto-fill back in.

Is there a "proper" way to split this file up? There are also page references for an interactive PDF, contents and index. If additional pages end up being added or moved during additional editing, will the references scattered throughout the book automatically update? I know I have to update the contents and index myself. 

I'm about to start another document for a separate book, is there a specific way to do this from the beginning, whilst using primary text frames? This document is going to be 100s of pages and all the documents have a lot of links and imagery. 

I've used documents in books before but never had to start or set one up myself.

Thanks!