I can't believe I haven't come across this post before—this is something I've been thinking a lot about, trying to figure out what the Tower is and how it ties to John's power source. I have also come to the conclusion that John might be using the perma-siphoned resurrected in his sunk-boat attempt at gathering enough energy to redo everything. I have a very long, half-written post somewhere in my drafts… This might very well be what upset Alecto so much and led to the tantrum that precipitated the whole chain of events where RBs showed up, lyctorhood was put into practice, John deactivated Alecto, fled to the Mithraeum, etc.
We get a thorough explanation of how siphoning works in the Avulsion chapter in GtN, and the mechanics of Eight siphoning are explained even though the siphoning used by Harrow in that trial is actually a different type, performed without sending Gideon's soul anywhere, and this stands out in a book that isn't in any hurry to explain its magic system and is selective about which puzzles we get to witness.
We are also told by Ianthe in GtN that there's a place “over the river […] where the soul goes when we knock it about… where the things are that eat us”, seemingly referring to Hell and not to the River Beyond. Augustine then says the Eighth house “sucks at [the stoma] like a grotesque teat”, confirming that the Eighth send souls to the stoma, and gets energy from there, not from the River per se? We then get of course a lot of info in HtN about the River, exerting space in non-space, etc. particlarly this interesting interjection by Harrow: “You can't build in the River! […] You might as well try to wall off time with bricks and mortar.” In Canaan resides “The sum of all necromantic transgression”, etc.
(Some of this has already been explored in u/Zealousideal-Sea9006's series of posts on what Cytherea was really doing in Canaan, which I absolutely recommend.)
Re: energy, one thing I'm fixated on—there's been very little information on what “spirit magic” is. For bone magic and flesh magic we understand thanergy and thalergy are used. There isn't a clear indication of what energy is at play when spirit is manipulated, or in what form would energy arrive from the River. There is no mention of “spirit energy” in all three books except once, Abigail speaking of Harrow's “heterogeneous soul situation” as her “spirit energy” being “diverse”. The entire River is, according to Augustine, made somehow of spirit magic, not spirit energy. As if this particular kind of magic was an extant, non-manmade thing, whereas bone and flesh magic are the specific practices. Are magic and energy synonimous in spirit magician lingo? What kind of energy is it that comes pouring from the stoma? I'm not sure how it could be thalergy or thanergy, seeing how nothing in the River is alive or materially dead—perhaps some earlier, primal type of spirit energy that converts to thalergy when materialized in our world? Is there meant to be a cycle, instead of an endless entropy of thalergy -> thanergy? Where does thalergy come from anyway? Energy can't be created, according to the laws of thermodynamics… (*)
The interesting thing is that Ianthe, who's been trying to figure out the Resurrection, isn't thinking about using human souls—she's thinking about using a displaced planet's soul. Tamsyn did say Ianthe was yet to do one massively shitty thing in AtN…
(*) My guess is: according to “Anastasia's tripod principle”, you need soul + body + thalergy for someone to stay alive. If someone's soul is separated from the body, but spirit magic is rushing into it, it might sort of fill the role of the original person's spirit, which allows the body to keep producing lots of thalergy which can be used outright or decayed into thanergy… and as we can see with Alecto, Jod or even Gideon, the amount of power/strength a body can generate and spend is bigger the richer the soul is, so an endless supply of spirit energy could mean a body produces much more thalergy than usual. There might be something weird going on with spirit -> thalergy conversion, considering babies release an immense amount of thalergy despite being small, no matter how much growth their cells are going through… as if their thalergy drew from some kind of lifetime reserve in the Beyond?