Lord Valentine's Castle is a very good book, but far from top class Silverberg. If you want to see him at his best, check out his early 70's books - like "Dying inside" or "The Book of skulls", both masterpieces, especially the former.

Anyway, the second book in the series "Majipoor Chronicles" is a short story collection, evach story with different characters and set at different time and place, does a great job of fleshing out Majipoor and its history. Some of those stories are really good, others not so much, but overall well worth reading, especially, if you liked the world of Mjipoor. On the oter hand, the next installments "Valentine Pontifex" and "The seventh Shrine" were disappointing. I haven't read the prequels.