I knew I wanted to put the otherworld stuff in here - more about the Danaan and Fomor, their plans, section on the Hobfolk, what's really going on with Cromm Cruach, etc. Stuff that's important for the GM in dealing with those setting elements.
There was a question about going broader, e.g. did I want to do more working up on the Druid Order, and maybe give it a book on its own. (I like the idea of just 'Order' as a title.) But that would involve a lot of new ideas/material. Looking at the rather low sales level so far, I think the best plan is to parcel up all the ideas/material I have already into one book, then it's all out there. So I can do short sections on blight creatures, crossing the Mistwall, etc in a section for Albion's secrets alongside Annwn's and Abred's. I like the triplicity.
The working title has been The Book of Willow for a while, but I'm heading toward The Book of Riddles (relating it to the Thread of that name). Softback, somewhere in the region of 96 pages. Out when it's ready. ;)