I measured my book and the measurements are:
Length: 270mm or 27cm
Width: 197mm or 19.7cm
Spine: 14mm or 1.4cm
Using a calculator I worked out the centre point by dividing 270 by 2. Then I drew a line using the line segment tool and positioned it in the middle of the document using the rulers. I made two more lines and positioned them 7mm to the left and right of the centre line. Then I selected all three lines and clicked View > Guides > Make Guide. The lines then become guides.