Briefly – You have to know if the book is to be printed or will be digital and, if digital, which format(s). Assuming it’s for print, you need to know the nature of the book. Is it, for example, a novel or is it say a heavily illustrated information book. If illustrated, will the illustration be integrated with the text or have a separate section. Will the illustrations be printed in colour or black and white.
You need to know the Trimmed Page Size (TPS). When you set up your New Document in InDesign you choose Print for the Intent, you add bleed to the TPS and add margins and possible columns and, choose Facing Pages and a Primary Text Frame. You will need to select and choose appropriate fonts for the kind of book. Set up a Baseline Grid, document grids. Set up Paragraph and Character Styles and possibly Object Styles. You set up folios on the Master Pages, set up Running Heads, decide on the styling of prelims, drop caps, chapter openings etc. You’ll need to flow the MS into the InDesign document, Place images. Decide on a cover design – is it paperback or case-bound, with a loose jacket?
You’ll need to liaise with the printer regarding the PDF spec for the output.
LinkedIn Learning has online video tutorials on how to do all of this – you can get 30-days free access.
Regrading templates, you might find something here, (collected by Redokun):
Derek, thank you very much. Very much on target for the start of a complex project.