I'm learning about desktop publishing and I've layed out a book in a desktop publishing application.
I want to use inkscape to create a cover.

From what I've worked out, my cover needs to have the equivalent width of 2xA4 documents, plus room in the middle for a spine.
People who do this work professionally use basic maths calculations to discover the measurement needed for the spine width.
As far as I can tell, doing this calculation is easy.

My next step would be setting up a document in Inkscape that has the dimensions I need and the guides that I need.
Creating the document at the width that I need is also easy.

The bit that I can't figure out how to do is this:
How do I set up guides that can help me distinguish the spaces for the front cover, back cover and spine?
 

The quickest way :

1. Open a new A4 document.
2. Enable snaping to corners of bounding boxes and page borders. Draw a rectangle and adjust it to page size using snapping.
3. Duplicate it twice. Set the width of the central one according to your calculations for the spine. Using snapping, place them all aligned horizontally and next together
4. Select the three rectangles then press ctrl+shift+r (to resize page to content) and then shift G (object to guides).