SPECIAL NOTE: As has been the norm for many years the CMMI Institute (ISACA) continues to evolve the CMMI eco-system, including the model itself, training material, the appraisal methods, related guidance, relevant policies, etc. Please monitor https://cmmiinstitute.com for the latest updates and material.
For an excellent collection of material from Carnegie Mellon University, Software Engineering Institute, the following link takes you directly to their browsable and searchable digital library.