There are a number of books written to discuss software development on Cortex-M. Examples include:
Arm offers a free of charge Arm Helium Technology M-Profile Vector Extension (MVE) for Arm Cortex-M Processors e-book.
Arm offers introductory Arm Architecture on-demand training courses, including M-Profile architecture, and appropriate development tools.
Coursera offers Arm Cortex-M Architecture and Software Development Specialization , a collection of online training courses.
Arm Education provides a number of resources to support STEM education at school and university level.
Arm offers instructor led training for organizations wishing for deeper technical training, including specialized topics such as Arm Helium technology and TrustZone for Cortex-M .
Arm also has a number of Approved Training Partners who can provide training courses to meet your needs.