IndianWeb has a short but useful article entitled What is the difference between a DAM and a CMS? It is neither long nor very detailed, but covers the essentials.
A little more on RFIs and RFPs: Know your DAM Requirements.
More on DAM procurement, MediaValet has published 2 short articles, DAM Procurement Considerations Part 1: On-Premise vs. Software-as-a-Service
Digital Asset News has a great and more complex article entitled Leveraging Intrinsic and Extrinsic Value: Understanding Why Successful Digital Businesses Are Cash Rich which is well worth a read!