"Well, if it's from another ghost it's on ghost stationery and then it's easy, but if it's on regular paper I can generally persuade the post owl to open it. Not that I get a lot of mail of either sort."

"The post owl."

"They're very clever little beasts."

"I guess they'd have to be. How is it ghost paper can be written on but ghost heads can't be completely severed for your posthumous convenience?"

"You know, I'm not sure. Perhaps it has to do with how killing parchment is more complicated than killing a person."

"That... sounds weird but a lot of magic here sounds weird to me and I don't have a better guess."

"Well, you're in one of the best places in the world to learn about it!"

"I suppose so!"

Sir Nicholas doesn't have much else to talk about; he makes his farewells and drifts off. 

Cam heads back to the library to look up post owls!

Post owls are sort of taken for granted by a lot of books; they tend to assume the reader is more interested in breeding them or handling their medical needs than with how they work. But he can learn that they carry mail and packages, have a magical ability to find any person or address that hasn't been magically hidden from them, and are intelligent enough to understand simple instructions like "get paid this many of this kind of coin before letting go of the package" or "wait for a reply". They're stronger and more resilient than nonmagical owls, but larger deliveries can still require a team of up to a dozen working together. International mail is often sent as a series of nested packages addressed to national post waypoints, where they can be passed on to another owl and where the first one can rest before returning home.

Golly. What an incredibly stupid and charming mail system. He looks at one of the books about breeding them in case it explains whether you need to cast spells on them or if they just hatch that way.

Breeding the original generations of post owls from ordinary owls was a complicated process, and even now new post owl chicks need to be fed certain potions at the appropriate ages to make sure they grow up healthy and strong and clever.

Adorable.

Now Cam wants to know about ghosts!

This book says ghosts happen when a witch or wizard dies with unfinished business. This book says ghosts happen when a witch or wizard dies who's unusually afraid of death and/or the afterlife. This other book says ghosts are more common in cases of murder but suicide, accident, and natural causes can also leave ghosts occasionally. A couple of them agree that ghosts will occasionally cease to exist, and that this process is at least partially voluntary on the ghost's part. They all agree that ghosts lose some of the living person's personality, and change much less than living people over time. It's very rare for a ghost to acquire new interests or move on from old grudges or, generally, grow at all. Small children almost never leave ghosts and this is generally regarded as a mercy.

Huh. This wasn't conversationally obvious but he wasn't specifically trying to ferret it out.

Right, time to stop being distracted by the weirdnesses of random folks he meets in the halls, and instead read Strong and Quiet Mind.

Occlumency is the discipline of protecting your mind from magic that might affect it. It's most directly useful as a counter to legilimency, a method of reading surface thoughts and memories, but it can also let one avoid the effects of the confundus charm (confusion and suggestibility), resist the effects of veritaserum, notice when one's memories have been wiped or modified, and may help with throwing off the imperius curse.

That sounds very useful! How does one do it?

The most basic kind of occlumency is concentrating on a single image or concept so intensely that a legilimens can't see anything else, but for that to work you have to know there's a threat. More advanced practice involves detailed introspection to notice when one's thoughts are being nudged by an outside influence, and the cultivation of mental habits that give thoughts more metaphorical inertia and make them harder to modify. There are exercises, starting with a simple "stare at an object and don't think about anything other than that object for sixty seconds" and moving on to more sophisticated introspection, concentration, and thought-redirection techniques, and some ways to tell if you're doing them right, but the only way to be sure is having someone try something on you, ideally when you aren't expecting it, and seeing if you detect and/or block it.

That doesn't sound too hard. Cam makes himself a nice malachite sphere to stare at and meditate on for a while.

It feels about like the book said it would. He can do all the exercises with similar ease if he puts several hours into it, though he will at some point in there get kicked out of the library.

He will relocate to his assigned room.

Then he can practice as much as he wants. Late in the evening he gets a note under his door inviting him to join Harry at their first occlumency lesson the next day.

He will show up for that if it deigns to specify a time and place! (He is now wearing a mechanical watch.)

It does; the time is after breakfast the next morning and the place is Dumbledore's office again (also the password is now apparently "blood lollipop").