I finished the War of Kings HC over the weekend. Wow, that was definitely worth the money! I already had Secret Invasion: WoK, the War of Kings mini and the WoK: Who Will Rule? one-shot.
However, with the HC, I not only got those in the bigger page size but I also got a bunch of issues I didn't read when they came out. Both Darkhawk minis are in there (those were FANTASTIC!), the WoK: Son of Hulk issue and the WoK: Warriors issues are also included.
Those definitely flesh out the main series and even help explain some events in Nova. Such as how Blastaar came to rule the Negative Zone and obtained the Cosmic Rod, how Blastaar agreed to side with the Shi'Ar and invade our universe, etc.