A short summary:
Workbooks show your issues in an Excel-like view where you can sort, filter and edit your issues like in Excel.
Workbooks can be created based on projects.
Deleting workbooks will NOT delete the issues that it contains.
In details:
Workbooks in JExcel are like tabs in Excel and provide a filtered view of your issues.
To create a workbook just click on the "+" icon (1).
After clicking on the "+" icon you will see this blank page.
On this page, you will have to click on the button "+ Workbook" (1).
They can be created based on projects.
How do workbooks look like In JExcel?
On the top navigation bar you can see a list of your workbooks. If you grab the workbook's name you can easily reorder them with drag and drop.
You can add new workbooks by clicking on the button + at the and of the navigation bar.