The Citadel (Mirror World Book #2) LitRPG series;

If you’re a warrior seeking glorious feats, we need you! The walls of the Maragar Citadel are always in need of heroes!
Or are you a wizard in search of forgotten lore? One visit to our Ancient Library will reveal you many a great mystery!
Or are you an online player craving excitement? Then you’d better hurry! The Maragar Citadel is rife with adventure - choose one to your liking!
Performing deeds of online valor is the last thing on Oleg’s mind, though. Neither does he care about any ancient lore. He’s never trusted adventure, anyway. Mirror World is no place for the likes of him. Still, he’s here to stay - at the demand of Reflex Bank which has granted him a loan for his daughter’s hospital treatment. Which is the only reason he’s joined the ranks of the defenders of the Maragar Citadel.