Until a year or so ago, I didn't know I loved Snape/Filch. But I do, and pitch-perfect stories like this one are why. It's meta and parody and believable and hilarious and poignant at the same time. (What -- those horrid, sexist, lie-back-and-think-of-the-Founders books were right??. And I love the way that romance novels are educational -- "Isn't that common knowledge?" haha)
Mrs B's book is exquisite (I own a similar one called What a Young Woman Ought to Know, circa 1905, complete with a chapter on "solitary vice"). And I love that oblivious Albus not only screwed things up, but that he's ultimately inconsequential to what ends up happening.
Argus's voice is excellent; his phrasings are so characteristic ("on account of. . ."; the "lad"). Severus is equally fine. slashpine's comment about their "combination of grubbiness and practicality. . ." is spot-on. ("No need to turn down sex in the interim" -- so Severus. And Argus's rational thoughts about drains are so right for the way you portray him.)
These lines made me laugh aloud with their humor, insights, and character revealing-ness:
It always works out for them in the end, and I'm not even wicked stormed out of the closet and slammed the door behind him before Argus could even tell him he had spunk on his boot. He was a tea man, awake or sleeping. I loved Argus's whole "trio of thoughts" here (and the way that the magic number three keeps informing the whole fic) a bout of hormonally charged dreams brought about by too much fresh fruit Haha! Well, if you're used to chips, beans, and toast, fresh fruit will kill ya. "Anyhow, they've always got hearthings in them, on account of the nice young ladies in them usually aren't keen on getting married off to wicked dukes and the like." Severus wondered if he might be having an aneurysm. Ahaha! The perfect pacing here, and the way the passage is packed with things wonderfully unsaid and undescribed. I can just see their expressions. "Himself might give us a honeymoon" / in a tone that suggested he was being helpful even if both his wheezing and bowleggedness were beginning to grow a little alarming
In short, if I may be blunt, Mystery Author (though I think I've got you sussed this time), I love your "wiles." A stellar fic.