Teen Book series of the MC being transported to different worlds to solve the puzzle

Question

I’ve tried to remember and search for this book series I read about 14–15 years ago.

From what I can remember, it had the main character being transported to different worlds with different environments for every book, and I believe the MC would have to solve how the world worked to leave.

There were the ‘denizens’. I don’t recall them being in every book, but one of the worlds had them as the main subject. They could of been in every world.

One of the memories I have is that the denizens were almost bland, or I pictured the world they were in being black and white.

It was a teen fantasy book. I believe the character had the power to manipulate time somehow with an item of some sort. Or there was a clock device that was important during one of the situations.

The series wasn’t completed when I was reading back in 2005. The series had at least four books. I am based in the USA and I believe I read it from the local library.

The MC was a male.

The denizens ignore his existence unless he did something significant to have them notice him.

Denizens seemed to be without free will for the most part.

One of the settings was in a town of the denizens.

There was a higher power that watched over the denizens and they were on guard against the MC when he would be in the different worlds

The higher powers weren’t aware of him in the early worlds, but later they started to recognize his existence.

The cover of one book having a device with multiple gold revolving circular rings with a black background.

Answer 1

I think this is Garth Nix's The Keys to the Kingdom series. The timing is correct; the series was published over the course of 2003–2010, so it was incomplete as of 14–15 years ago. The plot features magical clock-based artifacts that are used to travel around to seven realms associated with the seven days of the week.

Per Wikipedia:

The series' protagonist is Arthur Penhaligon. The series begins on a Monday, with the main events starting a week later on the next Monday. Each book moves onto the next day of the week, over the course of about three weeks in Earth time, concluding on a Sunday. Each day features beings, collectively known as the Trustees, who each govern a portion of the House, which is the center of the Universe. The Lower House, the Far Reaches, the Border Sea, the Great Maze, the Middle House, the Upper House, and the Incomparable Gardens.

In the beginning of the first book, Arthur lives a relatively normal life as an adopted child in a large and caring family. An asthma attack on a Monday that should have killed him brings him into contact with Mister Monday, who rules the Lower House. He eventually finds his way to the Lower House himself, where he is to find the cure to a plague brought to his world by dog faced 'Fetchers'. By convenience, he is declared Heir to the Kingdom and given the Lesser Half of the First Key, which is shaped like the minute hand of a clock. Because of this Key's magical properties, Arthur is relieved of his asthma while in contact with the Key or in the House, and proceeds to a strange and dangerous set of adventures.

As Arthur discovers, the Will of the Architect (creator of the House and the "Secondary Realms" that surround it) was not fulfilled as it should have been. Instead, it was broken into seven pieces by the Architect's Trustees, the self-named Morrow Days. The Will was forced to act on its own, and its First Part chooses Arthur to be the Heir to the Kingdom. It thus becomes Arthur's responsibility to recover each of the missing pieces of the Will, defeat each Trustee — each of whom has been afflicted with one of the seven deadly sins — claim their domains by taking each Key, and ultimately fulfill the Will.

"Denizen" is also a technical term used for some of the inhabitants of the various day realms, which the protagonist could be turned into if he uses his magical key too much.