They weren't written & published in chronological Pernese order.
But there is a general chronology. The main story takes place in the same time frame as the two you've already read. I'd recommend that you either find the ones that take place earlier, beginning with Dragonsdawn, which tells of the colonist's arrival on Pern, & Dragon's Eye, I think it is, & Nerilka's Story & Moreta, which are all 'historical' from that viewpoint, & read them before you go any further, or pretty much read them by publication date.
The first written were Dragonflight, then Dragonquest, which are the core of the main story, & fit just before/around the Harperhall trilogy/Menolly books. By the by, the third in that trilogy isn't really about her, it's Piemur's story. And The White Dragon falls just about there on timeline, too.