IAEA Medical Physics Handbooks
The IAEA has produced an exceptional series of medical physics textbooks. All of them are freely available to download in pdf form:
The IAEA Division of Human Health generates a lot of useful content for radiation oncology, diagnostic radiology, and nuclear medicine. These documents include reference material, guidance documents, and quality assurance guides and specifications. All of it can be downloaded for free.
IAEA Dosimetry and Medical Radiation Physics (DMRP) Publications - link