There are many books on QoS, I've read more than a few, and in my, not so humble, opinion, cannot recommend any if you desire to truly master QoS, either in practice or theory.
The best starting approach in truly understanding QoS, you really need to understand application traffic needs vs. traffic wants. Also, you need to truly understand application traffic needs and wants, depend on both the network protocol and "end users" for what's acceptable. Also, QoS can impact optimal bandwidth usage. I mention the foregoing because you wrote you want to master QoS
Cisco OS guides and command references often have sections explaining the devices QoS features. They also have White Papers, Technotes, Cisco Live presentations on QoS. (Actually, the forgoing contains excellent "facts" in what their QoS features do and their configuration options. As to guidance how those QoS features should be used or parameters changed, well, what they do usually won't make things worse, and can help some critical traffic such as VoIP.)
Usually, high end network devices offer "poor" QoS features, as they tend to support high bandwidth.
Likewise, in my (dated) experience, ISPs also ignore QoS and also push bandwidth.
Now Service Providers, though, also in the past, often did support limited QoS support, so you could run traffic, like VoIP, on a converged SP network where a potential customer couldn't be convinced why they needed a 10 Gbps link to replace their former T1, which met their needs while using QoS.
However, "cloud" services, have pushed a need for better QoS services.
So, what QoS need do you have for/within an ISP domain? (If confidential, if you don't want to be fully public, send me a PM.)
