This is a non exhaustive list (March 2021) concerning the web resource regarding the way to develop a custom bootloader for STM32. So please consult also the web.
- Mastering STM32 – it is a paid book that explain the STM32 but it has a chapter that talks about custom bootloaders, very well done.
- STM32Fx Microcontroller Custom Bootloader Development – is a video course (is not free) that deals with the bootloader, very well done.
- From Zero to main(): How to Write a Bootloader from Scratch
- How To: STM32 Custom Bootloader/Application
- Building a USB bootloader for an STM32
- STM32 Bootloader