1) Professional Embedded ARM Development, by Langbridge 
This one is more about ARM instructions than about bootloaders or embedded OSes, but it does cover bootloaders in detail with case studies of some relatively unknown embedded boards, of the well-known Raspberry Pi, and of modern bootloader software like u-boot.
2) Building Embedded Systems, by Gu 
This book is broader than the one above, covering all kinds of embedded systems. It's the better book to understand the big picture, and understand how all the components (bootloader, OS, SoC, hardware buses, storage) interact. It does cover bootloaders but not in depth and not with case studies.
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