ROS is a must have for CtL Changeling the Lost(CtL)is what's called a limited run. There are so many titles in the line and then it's done(but is it?). Anywho, ROS has a lot of new merits for the game and alot of clarification of the rules and ALOT of alternative rules for things in the Main Rulebook that needed fixing like the making of Pledges.
The big book of Changeling powers A must for the consumate Changeling fan. This is the glamour, powers book for the limited run Changeling rpg series (5 or 6 books I believe).
Imiginative Catalyst This has been by far the best non-core product I have ever owned produced by white wolf. 'Rites of Spring' introduces ideas for over all theme and light implementation. It caters to a Story Tellers need to tailor personal stories with suggestions on optional rules, motifs, and even guidelines for presenting description of the fantasy environment that the Lost inhabit. If you own the Changeling: The Lost book and feel as though you need an addition to be comfortable running the world then this book is an excellent tool to have available. The book also includes some suggestions on merits and alternate contracts to play with as well.