I haven't ever gotten around to writing a workbook for the middle school book.

In part this is because the IMLEM website has 500 pages worth of past contest problems with detailed solutions arranged according to the chapter numbers in my book. It's a great resource -- probably better than anything I would have written. And it's completely free!

See http://www.imlem.org/meetdoc/index.htm

For example, to find problems on the first topic covered in my book (Meet 1 - Geometry - Angle Measures) you can click on any of the number 1's you see and then scroll down in the PDF file until to come to Category 2 - Geometry. There will be a sheet with three problems arranged in difficulty order: the first is (relatively) easy, the second is medium, and the third is (sometimes very) hard. Each page is followed by a sheet giving the answers with explanations.