Best books/blogs/courses to learn advanced networking concepts?

At one point I had read most of the books out there on Multicast and found "Interdomain Multicast Routing" to be the easiest for me to grok the topic with useable examples from basic to the more advanced deployments. The book is old but I believe it is still very relevant.

https://books.google.com/books/about/Interdomain_Multicast_R...

I also got a lot of out "Optical Network Design and Planning" as an intro to designing WDM and optical networks. This one really helped me get a vendor neutral understand of modern optical networks. I found it to give me a good base understanding to communicate and understand vendors when reviewing their products (like Cienna).

https://books.google.com/books/about/Optical_Network_Design_...

Beej's guide is well known. I learned networking by hacking, reverse engineering. such learning materials are always so in depth and include bit-level protocol accuracy, widely unknown quirks and secret specifications.