Why didn't Snape keep or secure his copy of Advanced Potion-Making?

Question

Advanced Potion-Making (by Libatius Borage!) was used for sixth year Potions classes. Canon does not make clear whether it was also used for seventh year Potions, so I'm going to presume it was only the sixth years' textbook. Slughorn passes out two battered copies of Advanced Potion-Making to Harry and Ron, and Harry ends up with Snape's old textbook, as we know. A few relevant passages from Half-Blood Prince and Deathly Hallows:

Harry bent low to retrieve the book and, as he did so, he saw something scribbled along the bottom of the back cover in the same small, cramped handwriting as the instructions that had won him his bottle of Felix Felicis, now safely hidden inside a pair of socks in his trunk upstairs.

This Book is the Property of the Half-Blood Prince

_{Half-Blood Prince - page 183 - Bloomsbury - chapter nine, The Half-Blood Prince}

‘SECTUMSEMPRA!’ bellowed Harry from the floor, waving his wand wildly.

Blood spurted from Malfoy’s face and chest as though he had been slashed with an invisible sword. He staggered backwards and collapsed on to the waterlogged floor with a great splash, his wand falling from his limp right hand.

_{Half-Blood Prince - page 489 - Bloomsbury - chapter twenty-four, Sectumsempra}

‘You dare use my own spells against me, Potter? It was I who invented them – I, the Half-Blood Prince! And you’d turn my inventions on me, like your filthy father, would you? I don’t think so ... no!’

_{Half-Blood Prince - page 563 - Bloomsbury - chapter twenty-eight, The Flight of the Prince}

‘Snape?’ shouted Harry. ‘You didn’t say –’

‘He lost his hood during the chase. Sectumsempra was always a speciality of Snape’s. I wish I could say I’d paid him back in kind, but it was all I could do to keep George on the broom after he was injured, he was losing so much blood.’

_{Deathly Hallows - page 66 - Bloomsbury - chapter five, Fallen Warrior}

There is no question Snape is the Half-Blood Prince, or that Harry got hold of Snape's former textbook; as well as useful potion-making information, it also contained rather questionable (Levicorpus) and downright dangerous original spells (Sectumsempra).

Not only did Snape have the opportunity to secure his copy of Advanced Potions Making at the end of his sixth year, I'm guessing he had all of his seventh year to destroy, hide, or keep the book as well. Also, he had at least five years as Hogwarts' known Potions master to ensure the book was kept away from the students.

As Potions master, Snape would have had access to all inventory relating to the Potions classes, including Hogwarts' copies of any potions textbooks. It would seem likely, while arranging or cleaning his classroom and stores, Snape would have come across his old textbook at some point. I personally think he would've recognized it. So, to ensure the book didn't fall into students' hands:

Why didn't Snape keep or secure his copy of Advanced Potion-Making?

(Or did Snape want a student to find it for boastful personal reasons, just as Tom Riddle used the diary Horcrux to take credit for the Basilisk and being the Heir of Slytherin?)

Answer 1

The book was probably left in the cupboard by Snape as a personal reference, and found by Slughorn when he took over the class. Of course, after many years of referring to it, Snape probably knows everything in it completely from memory, and he doesn’t need to look at the book any more.

Harry and Ron only get textbooks from the cupboard because they didn’t buy copies beforehand, which is fairly unusual. When Snape was running the course, you’d know whether you were taking Potions to NEWT before you started the class (and Snape isn’t the one to make exceptions).

It sounds as if Slughorn expects them to borrow them, then return them when they have their own:

“Ah, yes, Professor McGonagall did mention… not to worry, my dear boy, not to worry at all. You can use ingredients from the store cupboard today, and I’m sure we can lend you some scales, and we’ve got a small stock of old books here, they'll do until you can write to Flourish and Blotts….”

Prior to Slughorn taking the position, the copy was quite secure.

Somehow I don’t imagine Snape being so generous. You turn up without the textbook? Tough, five points from Gryffindor. Ten for talking back. And another fifteen at the end of the class when you can’t finish the potion.

There was probably never a risk that a student would get that book in one of Snape’s classes, and the possibility only opened up when Slughorn took over the class.

I imagine that in earlier years, Snape would have been more careful, but he has a lot on his mind – the ongoing war with Voldemort, his Unbreakable Vow, and preparing a DADA course – an old notebook that he hasn’t looked at in years probably isn’t top of his mind of things to remove when sanitizing his old classroom.

Answer 2

I think you may be overestimating the value of the text in question...using a muggle's perspective.

To Snape, it's probably just an old textbook he took some notes in, long ago. He wouldn't seek to secure it any more than you'd secure your old 11th grade social studies textbook. His own personal knowledge of potions, by that point, would have vastly exceeded that which he possessed as a 6th year student.

It's useful to Harry, because Snape was an excellent potions student - but copying the best student's notes is generally valuable, for students. If you're not a student, that value might not be as obvious or apparent.

Answer 3

In addition to alexwlchan's answer: I think that Snape's reason to let the book in the cupboard was that it is a part of the Potions class he shared together with Lily. The final fall-out with Lily happened after the OWL Defence of Dark Arts test, so the first four years they were together as friends. While Lily (and anyone else) is not mentioned in the book, it is a powerful reminder of old times.

Snape never got over Lily's death and is therefore prone to hold everything which has a connection with Lily:

Snape took the page bearing Lily's signature, and her love, and tucked it inside his robes. Then he ripped in two the photograph he was also holding, so that he kept the part from which Lily laughed, throwing the portion showing James and Harry back to the floor, under the chest of drawers...

So while not explicitly mentioned, I find it easy to imagine that Snape, when alone in the Potions dungeon, opened the book again to refresh his old memories. That he is particularly vulnerable in this regard indicates his extreme precaution to store away his worst memory, the fall-out scene, from Harry's reach.