im not the best person to ask ,,, im assuming u mean subahibi, tsui no sora and its remake . all of the games r self contained so u dont NEED to read one to go to the other , but i think reading subahibi is a good starting point, its the longest out of them and basically a reimagining of the og tsui no sora . after that theres tns and tns remake but idk which one of em is better to read first (currently only the remake is translated but the translation for the og is pretty close to being finished)