You can create several master pages within one doc. There you can change the margins and lots of other things. Then you can apply these masters to your "normal" pages.
For the cover, I would advice to leave it apart, since you created a spine, but if your book is only going to be 20 pages the spine can be omitted. In this case, first page of your doc is the cover and the last page is the back.
It seems to me it's a saddle stiched booklet, so if "print ready" means imposed, you'll have to create a booklet. Make sure the complete book has 20 pages because if you print on both sides of the paper (let's say you made the doc on A4) the imposed booklet will be printed on A3 format (double A4).
You can print it in that way by clicking the "file" menu then choosing "Print Booklet". From the "Print Booklet" dialog box click on "Print Settings...", then from the "Printer" drop down list choose "Adobe PDF" if you want to view it on your computer or choose your installed printer if you want to print it. Notice that pages 3 and 18 will be facing in "Preview" instead of 1 and 18 since the cover and it's back are counted as pages.
Note that printing direct to PDF requires that Acrobat Pro be installed so that you have distiller and the virtual printer, and is not available on Mac Snow Leopard or later. For those systems, see InDesignSecrets » Blog Archive » Acrobat’s Adobe PDF Printer Replaced in Snow Leopard