what does "mere obsession" mean here?
The transparent walls became again opaque, the old man’s vaticinations mere obsession. The street must have been empty, as it was now; the eye deceived.

Q: what does "mere obsession" mean here? what's the meaning of "the eye deceived" here?But perhaps rather it was the boundless sequacity of love that deceived, and most of all in the matter of the time which had passed since the bells stopped ringing; for suddenly Phrynne drew very close to him, and he heard steps in the thoroughfare outside, and a voice calling. These were loud steps, audible from afar even through the shut window; and the voice had the possessed stridency of the street evangelist.
‘The dead are awake!’
The above is the two following paragraphs. give the context, I'm still a little confused with "mere obsession" and "the eye deceived". Besides, what does "most of all" here mean?

Question

what does "mere obsession" mean here?
The transparent walls became again opaque, the old man’s vaticinations mere obsession. The street must have been empty, as it was now; the eye deceived.

Q: what does "mere obsession" mean here? what's the meaning of "the eye deceived" here?But perhaps rather it was the boundless sequacity of love that deceived, and most of all in the matter of the time which had passed since the bells stopped ringing; for suddenly Phrynne drew very close to him, and he heard steps in the thoroughfare outside, and a voice calling. These were loud steps, audible from afar even through the shut window; and the voice had the possessed stridency of the street evangelist.
‘The dead are awake!’
The above is the two following paragraphs. give the context, I'm still a little confused with "mere obsession" and "the eye deceived". Besides, what does "most of all" here mean?

Answer

Hi, For the sake of understanding the whole context, (sorry if it's too long) i will explain the sentence:
"The transparent walls became again opaque" To be transparent = to be able to see through it. Glass is transparent. Opaque = means something that you can't see through. Transparent and opaque are opposities. They usually are defined by 'how light is reflected', for instance something transparent = light passes through it (you can see) vs. opaque = the light doesn't reflect or pass through (can't see a reflection).

"the old man’s vaticinations mere obsession" = vaticinations = prophecies, to believe something will happen in the future, predictions. mere obsession = 'mere', in this context is referring to something 'insignificant' 'not worthy'. For instance, "he is not a man, but a mere boy". You could read it as 'being nothing more than (something). Morgan is correct too, 'something that goes by quickly/ fleeting"
So, mere obsessions = an "insignificant obsession" = "means nothing" mere = usually a negative meaning. 
Obsessed = to be preoccupied, you can't stop thinking about something ("I'm obsessed with her beauty").  
"the old man’s vaticinations mere obsession" = (LIT) the old man predictions, (are based on nothing more) his obsession (basically, making predictions is a little hobby).

Answer

an English novel named "Ringing the Changes"

Answer

What are you reading?

Mere obsession is like saying it was 'just' an obsession. It could be fleeting, like one that comes and goes.

The eye deceived=the eye tricks the man into thinking something was there?