I would start with the very basics of HPLC theory from a general Analytical Chemistry textbook. However, this will really teach you very little about proper day to day use of modern instruments. That's something that really doesn't go in a textbook. If you've got some money to spend, LC Resources gives very in-depth courses, in-house, or at an event they host, or (nowadays more and more) real-time online. But the courses are very through, and a little expensive. Maybe the beat way to learn what's expected of you from this position is to find the instrument expert -- there must be at least one -- and ask if you can shadow them as they take the instruments through their paces.