I actually quite liked Asylum, it may not be among the best of the BBCs Doctor Who books, but is certainly an enjoyable adventure in the past. My largest complaint is perhaps the way that it starts out more like a modern historical than the traditional style - aliens in the past, rather than the purely historical Hartnell stories. Yet within 20 pages all this is seemingly forgotten, instead we end up with just a historical whodunnit (although a pretty good one!). Likewise at the end once the mystery has all been resolved some tedious and pointless philosophy ensues for absolutely no reason. Quite well written really, even if a little unoriginal. Something I had no problem with was the bringing back of Nyssa, I thought she worked quite well in the story - but why the fourth Doctor was used instead of the fifth I'm not so sure of. Yes it allows us to see an older Nyssa, but a later Doctor would save all the time paradox complications.
I didn't bother to read the essay at the end, it looked rather long and limited. From previous reviews it seems I missed nothing.
It does perhaps seem that Davill-Evans had a great short story idea, but failed in beefing that out into a standard Past Doctor Adventure. A fair enough read, quite an interesting Historical story, but it is lacking somewhat. The writer seems to have changed direction with the book several times, leaving some points very interesting, some points rather weak.