The interesting thing (to me) about the first Dirk Gently book is that it was largely a repurposed Dr Who script - in which version Reg was a retired time lord. Such a fun angle on the story!
Heh, which reminds me, it's my head-canon that the sofa in LtU&E is the same one that's in Dirk Gently and in City of Death. Given that it's flotsam in the space time continuum it seems perfectly reasonable :)