How do you get InDesign to automatically create pages from the Master to autoflow long docs? I usually work on magazines and brochures but I’m trying my hand at books. I seem to remember that Quark does this, but can’t figure out how to make InDesign do it. Also (cheeky second question) can you recommend a plug-in to get InDesign to create imposed proofs for a variety of sheet sizes?

You can use Autoflow by Shift-clicking when you import your text.
This will add pages and frames until all text is flowed into your document.