Hi Peter,
I don't have Kindle and I don't really read that sort of genre, but I've taken a quick peek inside.
Things that stand out for me. Firstly, it says first published May 2017 in the copywrite, so even if you have only just published it on Kindle, you clearly haven't just finished writing it as per your OP.
Secondly, the standard of writing is good. I couldn't see lots of SPAG mistakes, as often you do on the self published ones. There's a couple I spotted on first read through, but only one that made me wince. 
It's a little bit too flowery in parts with the descriptions, but that's partially my taste.
However the dialogue is stilted. "Hello Cynthia. It has been quite a while since I visited hasn't it? I've been quite busy over the past few weeks you see..."People generally don't speak like that. It's also in the same tone you're writing the rest of it.
"Good to see you Cynthia. Been a while since I visited, I've been so busy..." would be better.
But make it even more informal.
"Hi Cynth. Sorry it's been so long, manic at work..."
It's also quite confusing who's speaking. Give some dialogue tags. Not necessarily "I said"/"Cynthia said," but use some movement to give a clue who's speaking. "Cynthia brushed her hair back with an impatient movement." "I turned away, and my eyes were drawn to the peeling paint below the windowsill".
You can use them to give more clues to the place and the characters.
You have a tense issue in the "as I walk through the familiar hallway" paragraph. "As I walk through is present tense. You then have "he was a menacing looking character"-past tense.
(You can also lose the word familiar. You've already let the reader know he's been there before.)
I like the chapter names. It's a pity not to give the one different a colour too in some ways. Unless you need that chapter to stand out against the others, in which case that works.
On self-publishing/publishing, I'm not certain you're correct. My understanding is agents don't like it to have been self-published. I think the argument is: Either it sells on self-publishing or it doesn't. If it sells, then you won't want them (as they take a commission) or if it doesn't then they'll probably struggle to sell it themselves. I know you'll feel that isn't true as that getting it a publishing house who can get it in a bookshop etc.
My understanding is that if you have a reasonable following from a self-published novel then that can be an added catch for an agent when pitching your next novel, as they see that you may already have an audience to sell to. So if you're pitching another novel, then it may be no problem (although they may request it's taken down as a condition of contract), but I think pitching one you've already put out there is a no no.
I may be wrong there though, and others, more knowledgeable than me will be able to advise you on that.