There's 6 and 7 planned too, might be a bit of a wait. It's a real shame he isn't likely to finish the series, I would really be interested in that final volume (compilers).
The website notes that 6 and 7 are "only if the things I want to say about those topics are still relevant and still haven't been said." This does make me think that he isn't particularly planning on working on these books at all, especially given that the timeline goes "volumes 4 & 5, revise volume 1-3, reader's digest 1-5, then maybe work on 6 & 7"