print design - What are the basic rules for designing a book (novel)?

I promised my friend I'd design a book for him he's releasing for free and I could use the help of you guys.

I'm going to use the latest InDesign. The book should be in .pdf format and is intended for printing.

I googled as much as I could and this is what I found out:

• the project size should be 6'' by 9''



• the best font to use is Adobe Garamond Pro for text and Helvetica for headers


• outside margins should be 0.5'' and the inside margin should be 0.75''

I don't know what font-size/line-height to use for text.

I think these are the basics I found out. Can these be improved on? What else should I know before I start? Any recommendations?

Also, I live in Europe, where we don't use inches. Should I still set the sizes to the inch values, or should I somehow use cm/mm ? Because 6'' / 9'' does not convert to a nice cm/mm value.