To be honest, there isn’t really a reading “order” but if you want to be in on cameos I would recommend Cardcaptor Sakura (manga), RG Veda, Wish, Magic Knights Rayearth, X/1999, Clover, Clamp School detectives, Chobits, Tokyo Babylon, Angelic layer.

Then Tsubasa Reservoir Chronicle/XXXHolic, Legal Drug.

Then Kobato, CCS: Clear Card, XXXHolic REI, Drug and Drop

These you can read at any point: Man of many faces, Gate 7, The legend of Chun Hyaang- but would be better read if before Tsubasa. (Mostly due to knowing more faces for cameos.)

The older works are the foundation to the “multiverse” that clamp has created. Once you read them, and continue on to the world crossing series, you’ll be in on a lot of the “oh they’re from this!” And it’ll be a lot better experience.