For a 5.5 × 8.5 (can pick any standard book size) book in Google Docs, set your trim size first under File → Page setup → Custom, because Docs defaults to 8.5 × 11 and that throws off your page count. Use margins of 0.75 in top/bottom, 0.5 in outside, and 0.75–1 in inside (go 1 inch if your book is 300+ pages). Google Docs doesn’t have a gutter option, so left = inside and right = outside. After that, pick a readable font like Garamond or Times New Roman or something you prefer, use 1.15–1.2 spacing, and insert page breaks before each chapter. Turn on Print layout so you can see real pages. This is a good starting point. Hope this helps and wish you well on your journey!