After leaving Lockwood & Co. at the end of The Hollow Boy, Lucy is a freelance operative, hiring herself out to agencies that value her ever-improving skills. One day she is pleasantly surprised by a visit from Lockwood, who tells her he needs a good Listener for a tough assignment. Penelope Fittes, the leader of the giant Fittes Agency wants them--and only them--to locate and remove the Source for the legendary Brixton Cannibal. They succeed in their very dangerous task, but tensions remain high between Lucy and the other agents. Even the skull in the jar talks to her like a jilted lover. What will it take to reunite the team? Black marketeers, an informant ghost, a Spirit Cape that transports the wearer, and mysteries involving Steve Rotwell and Penelope Fittes just may do the trick. But, in a shocking cliffhanger ending, the team learns that someone has been manipulating them all along....

WHO'S THE MANIPULATER ?
I personally think it's Holly Munroe or it could be a new side character in book 4 that we'll see

What's your prediction?

To have been manipulating the team the whole time, they would have to be present the whole time, even if only as a side-character. Let's look at the mayor recurring characters aside from the trio.

Inspector Barnes: DEPRAC employee, deals with L&co in every book, is the closest thing they have to a handler, hates L&co's guts by self-admission. Isn't very high on the command chain as we see in THB, has no reason to manipulate L&co except to lessen his headache when dealing with Lockwood's 60Watt smile.

Quill Kipps and entournage: Fittes agents, rivalry with L&co due to personal quarrels with George and Lockwood. Starts out hating the team but forms a grudging partnership over the course of the books and is even lower in the chain of command than Barnes and seems to have undergone a revelation as to the expendency of agents. Manipulator? Unlikely, more prone to outright agression than sneakyness.

Penelope Fittes: Young leader of one of London's most prestigious agencies, rules her agency with an iron fist. Has appeared in direct contact with L&co in all three books, is involved in the Orpheus society maybe even the leader. According to the blurb, is the one who hires the team for a case despite having an agency of her own. Manipulator: very likely but may just have her own agenda separate from manipulator.

Rottwell: Leader of London's other top agency, is mentioned often but appears seldom. Not much is known apart from treating his agents like crap and being very paranoid. Lockwood doesn't seem too fond of him, is also in the Orpheus society. Manipulator? Could be, maybe acting through a third party.

Florence Bonnard: Relic hunter and Queen of the river. Appears only in the last two books but has mantained a long-standing friendship with Lockwood. Has contacts with all relic-hunters and half of London's dubious population. Willing to help out L&co forthe price of licorice and/or sugar with coffee, rumored to be a former rottwell agent. Manipulator? Could be, agenda unknown.

Mystery swordsman: Appears in both the second and the third book, rich and in high circles, very good with a rapier, collects only the best sources, seems to have kept an eye on L&co. Mystery inside a dilema, next to no information. Manipulator: Possibly, too many variables.

Holly Munro: Former Rottwell agent (am I seeing a pattern with Rottwell agents here?) and scarily competent assistant to Lockwood and co in THB. Similar past to Lucy and came recommended by somebody, meticulous, conscious about self-image, reduces the teams stress levels by 200%, helpful and seemingly the perfect assistant. Manipulator? Possibly, but is relatively new to the dynamic and has no mention in the previous two books.

The Skull in the jar: Type three ghost of uknown name and dubious procedence. Assisstant to Bickerstaff and possibly helped to create the bone glass, adores his former master and is obsessed with death of others. Wanted to come back to the land of the living for unknown and suspicious reasons. Can communicate with Lucy only and is afraid of loneliness. Has a skewed moral compass but is willing to help out Lucy after a bit of blackmail. Manipulator? Very likely but seems minimally fond of Lucy (the jilted lover). Most likely the only adult at Portland Road 35 even if behavious is childish at times.

Feel free to add other characters!