Complex sentence: After finishing both books, a third book can be accessed, although it is necessary to save the games'progresses with a Back-up RAM cart, since the Sega CD's internal memory isn't enough to save both games data.
Split into simpler sentences: After finishing both books, a third book can be accessed. Due to the memory limitations of the Sega CD system, saving progress made in book 3 must be saved on a back-up RAM cart since the Sega CD's internal memory isn't enough to save both games data.