I've created a Google spreadsheet for just that. This is a public document, so feel free to copy it for your own use.
Using this document, you enter the following each month:
- Total Income
- Total Sales
As you enter these, you will automatically see the statistics for the following:
- Yearly Income
- Yearly Sales
- Per Month Income
- Per Month Sales
- Per Sale Income
You can access this file here: Book Sales Statistics Spreadsheet
Currently, I have spaces in there for sales during 2012 and 2013. I kept these on the same page of the document for quick and easy comparison.
If you have any suggestions for improving this form, let me know in the comments.