The pocket is also the one that will stay in print the longest. Sometimes this version goes through various reprints, all with different covers. This tends to create some confusion, as people see a new cover and think it is a new title. OP A book can't be available indefinitely. Markets and tastes change after a while, so publishers may decide not to reprint a book.
Novels last two to three years, depending on the author.
Well known authors can last much longer. If that happens this month, you could be lucky to find one of the last copies on our shelf. But if it went out of print some time ago, you are out of luck. Only specialty stores that deal in hard to get books could be able to find the title for you, but expect to pay large amounts of money for this. Though I must admit that I too make use of the info listed on internet pages, I always keep out a wary eye.