You can download the books you buy with credits, and also anything currently in the Plus catalogue. I’ve found this very helpful as they remove things from the Plus catalog with very short notice periods, so you may not get to finish books you start. (And I remember stuff like how Salman Rushdie books were removed when he was in the news from the assassination attempt. Maybe that was publishers seeing dollar signs not Amazon, idk, but it was gross. Same when Sean Lock died.)