Practical Workflow for SAP, 3rd Edition (June 2014).
The SAP workflow bible. Two of our consultants were honoured to be co-authors of the 3rd edition.
Note: all profits go to Doctors Without Borders.

How to pass the SAP ABAP Certification Exam (June 2016).
Demystify the SAP ABAP certification process and get the ‘inside’ knowledge you need to pass the SAP Certified Development Associate exam. Includes 120 brand new sample questions!

SCN Articles

Workflow problems? Read this first!
The top 10 mistakes made by workflow beginners
More common mistakes made by workflow beginners
How to logically delete workflows
10 hot tips for passing SAP certification exams

ASUG (American SAP User Group) Presentations

Workflow as ABAP++
Presented in Orlando, Florida (2014).