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?)