I'd say they were common phrases. Skin Game might be the least common, but other than that one, nobody has to guess at the meaning of each book title. I'm not sure that "Downer Ending" is common enough knowledge to be both a known phrase and also an illusion to the events of a book about a wedding and manhunt.

And besides, it'd be too much of a spoiler. He couldn't name Changes "SNIPER BULLET". 

Most of these aren't really idioms or common phrases, though, right? I'd say Cast Away, Court Cases, Lady Love are, but the only one of those that gets close to plot adjacent is Lady Love, and that'd have more to do with Molly than Lara. I could see Court Cases being about issues between the courts, a la Lara and Molly, but Molly is playing ball as far as we know.

Harry isn't an idiot.  He's up against Things that have been around for much longer than he has.  Some of them were around when being able to read was a rare thing.  The supernatural world is survival of the fittest, and he's up against Things that are the smartest, and most dangerous because if they weren't, they would be dead.

Harry hasn't bothered to learn anything new or grow since pre-Changes, so no, I don't see his starting now.

If anything, he'll start leaning into being a thug even more, since everyone out-skills him so much that he can't hope to catch up, so better keep developing his Thews instead.

The man is the living embodiment of "Fight Harder, not Smarter"

The were wyldfae at that time.

I'm really hoping Ziusudra is the founder and leader of the Librarians. I think he'd be perfect for it.

• He's immortal.
• He's human with no known powers.
• Historically, he helped humanity survive a divine-based apocalypse.
• He was the last of a line of Sumerian kings.
• Sumerians all but invented civilization and writing.
• Two of the oldest surviving texts mention him by name. One is a guide from his father on how to rule; the other is the Epic of Gilgamesh, where he's mentioned as an immortal that Gilgamesh seeks out in his own quest for immortality.
• He could be an amalgamation character in the Dresden files, and Harry could meet him and learn at the end that he's also known as Atrahasis, Utnapishtim, and Noah.
• As an immortal, he could have overseen the creation and relocation of the greatest libraries in history, moving a collection from place to place, until it ended up in America.
• He could have overseen the growth of the earliest empires, and the wonders they built, from the gardens of Babylon to the pyramids of Egypt to the lighthouse of Alexandria.
• The library could be the latest incarnation of a long list he oversaw, including the libraries of Ashurbanipal, Alexandria, Pergamum, Trajan's Forum, Constantinople, the Baghdadian House of Wisdom, and finally the museums of the west.