If you want to focus on real-time, I would recommend other books. The one mentioned has no dedicated treatment on real-time. This one is interesting and very detailed down to machine code, but I didn't finish reading it yet: Embedded and Real-Time Operating Systems. This one I partially read some years ago and think it's very good (still subjective): Real-Time Embedded Components And Systems: With Linux and RTOS. I assume you know that the authors of FreeRTOS have also written some books which can be downloaded for free from their website.

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