Medpac publishes its annual, 200-page handbook of healthcare spending. This is one of the major go-to resources for the field.
https://www.medpac.gov/wp-content/uploads/2023/07/July2023_MedPAC_DataBook_SEC.pdf
Read about the history of MEDPAC and the history of many other aspects of Medicare in a 2015 artile by Kinney - here.