They key here was to remember the Nightwalker Thesis: spiders sometimes see better out of their butts than their faces. This one sure does.
spoiler:The problem is that you have a light on the left that needs to be frobbed, and you have a door on the right that needs to be entered. I found I was completely unable to cross the corridor in the light/door section of sewer; I was spotted every time. Also, I was unable to creep far along the right-hand wall without being spotted: due to the way the spider was positioned, he could see the right wall better than the left.
So I decided to ghost in two stages, one to frob the light and the other to get into the door.
Stage 1: Let's call the section of sewer before the light/door section Sewer-1, and the light/door section itself Sewer-2.
I started in Sewer-1, pressed into the corner on the left -- where the spider could not see me. I crept around that corner until I could see the spider (and he could see me, but not well enough to react). From that moment, I would only move in the brief intervals when he spun to face me, turning his butt to the opposite wall.
I crept around the corner to the left, moving as above. Then I crept along the left wall towards the frobbable light. When I started getting close, I would save, wait until the next time he turned towards me, then stand. I wouldn't move. Then, when he spun to face me again, I would lean and see if I could frob the light. Since it was still out of reach, I would crouch again; then the next time he spun to face me, I would creep forward... then go through the whole standing-up routine again, saving each time.
Eventually, I managed to work close enough that I was able to frob the switch. Now I had to return. Using the same method of waiting for him to face me, I tap-tap-tapped my way backwards (so I would watch him spin) into the lower-left corner of Sewer-2. Save. Then I rotated about forty-five degrees, so using the right-arrow wouldn't move me too fast. I crept a tap at a time into the doorway between Sewer-2 and Sewer-1, then around the corner into the Sewer-1 corner where I had started.
Stage 2: The secret door was open now, and I was hoping I could use it as a shield. I darted across to the right-hand corner, still in Sewer-1. Then I used the same technique as above to get around that corner into the lower-right corner of Sewer-2. I continued to creep forward along the right-hand wall... and I as I got to the section where I was spotted when I tried before, I noticed the open door was starting to interfere with the spider's line of sight.
I watched his legs under the door and continued my tapping technique, moving only when he was facing me. Eventually, even his legs were invisible. At that point, I just moved normally (still crouched), and he didn't alert... I guess the rule is if I can't see him through the door, then he can't see me (unless the author deliberately makes the door out of one-way glass!)
In any event, I made it to the doorway itself without an alert. There, I stood up, held down the shift key, and pressed the forward- and right-arrows simultaneously, which is the strongest Garrett movement. I had to wriggle a bit, but it was just barely enough to get me up and over the little step without having to jump (which probably would have alerted Mr. Spider).
I shut the door (via the switch) and opened the cell door, then proceeded as before.
Thus, so far I have an honest ghost going. Maybe Murder In Featherstone will turn out to be ghostable after all. Of course, I don't know what's coming up...!
At the moment, I'm in the small, locked room just before the cell; I had to pick my way in here, but I can't find anything inside. Is there anything here? If not, I can just move on.
Anybody? Is there anything to be found in that locked room just before the cell with the body and key?
Thanks,
Dafydd