Wait, What Was Their Plan Again?
The stolen pages, we find out later, contained a ritual to awaken an ancient evil named Dormammu. The rest of the movie revolves around hunting Kaecilius down and thwarting his plan to use Dormammu's power to create an immortal (albeit hellish) existence for all of humanity. Seems straightforward, as villain schemes go.
Doctor Strange, meanwhile, begins his Hero's Journey at the temple, and in an oddly clumsy scene, he happens to be strolling around the Masters Only section of the library, where he grabs a totally random book off the rack. He then casually flips it open to see that pages are missing. That's right: Of all of the hundreds of books in the library, he happens to grab the exact one that's key to the entire plot. They couldn't have had it off in its own special case or something, so there'd be a reason for it to draw his attention? Whatever, it doesn't matter.
Strange is informed by the grumpy librarian that the book is a series of spells that can manipulate time. The librarian then tells Strange that he'll murder anyone -- including Strange -- who attempts to steal one of the sacred books on his watch. But that is no match for the plucky doctor! Strange is intrigued, and learns just enough magic to open mini-portals to steal three forbidden books right behind the librarian's back! It's an adorable bit of magical slapstick ...
... Wait. Stop the movie.
WHY THE FUCK DIDN'T KAECILIUS DO THAT?!?
It Seems Like Stealing The Pages Undetected Sure Would Have Helped
From the start, the protagonists know exactly what the villain is doing, because he stole precisely one spell. They know carrying out his plan means attacking certain cities (where other temples are located that grant Earth magical protection), and so organize their strategy around it. Boy, shit sure would have been easier for Kaecilius if he'd managed to steal those pages without anybody knowing about it. He probably would have had enough of a head start to summon Dormammu before anybody caught on. If only there was some way to quietly steal books behind the librarian's back. Some method so easy that even a brand-new recruit can master it ...
"But they'd notice the book missing!" you might say. Sure, but that book was packed full of time-based spells -- they'd only know that the book was missing, not who took it or what the thief wanted with it. Or he could have stolen the book through a tiny arm-portal, ripped out the pages he needed, and then put it back in its spot. Nobody would have noticed until the next time they cracked it open, which would have been when Strange did it a few weeks/months/whatever later. Or, if Kaecilius wanted to leave no trace at all, just make a copy of those pages, then return the book.
"We can open portals to anywhere on Earth ... except FedEx Office."