Book or series with 7 layers of worlds, where each layer's inhabitants are like gods to the layer below

Question

Recently I've wanted to re-read an old series, however I cannot remember the name of said series/any books. I will provide all the details I can, and will hopefully be able to answer questions as well.

In the series, someone from Earth is transported to this other realm, once there, it takes them a bit of time to get used to everything, but eventually start taking part in the world. From what I recall, there are 7 (or 9) worlds (moreso realms), each one above or below the other. The people in each world above are gods to the inhabitants of the world below. This seemed quite like Norse mythology. The one the main character is in is much like medieval Earth, the one above similar to Valhalla, and the one below had sort of... spirits/elementals.

In the series, the main character eventually becomes a knight, and learns to fight. There are also giants, which as I recall were a major plot point in at least one book. He also at one point has to cross a bridge, and so fights a knight who would not let him past. I cannot remember his name, or really any of the characters names for that matter.

The writing, as I recall was kind of weird as well, sort of in first person, but occasionally using a narrator. The main character became quite powerful as well.

One scene I remember quite well, is the main character and a creature from the realm below (his servant) end up going up a long staircase to fight something from one of the realms near the bottom. I believe it was a dragon, and a powerful one. I do not recall if they ever get to the top, as some harpies harass them on the way up.

I read the books about 10 years or so ago, but I cannot remember if they had been out for long by then or not. I also can't remember the exact amount of books in the series. I borrowed them from the library, so no luck hunting through my old book shelves.

If I can add any clarification, or help in any way, feel free to ask. I've never asked one of these, but am generally impressed after reading through many others how quick people solve these.

Clarification below after reading Mithrandir's comment:

Series of novels, my best recollection was they were 700 pages~ or so. Read them in hard cover.

Read them likely around 2005-2007~ around there. As I recall most of the series was out, but still had a few books remaining? Most of the books were more or less loosely tied together.

Theme mostly seemed about getting by and surviving?

Plot was mostly about the main character becoming famous and making an impression on the worlds. The main character almost dies at one point, and then the gods from above (Valkyries, and the all-father) need his help fighting in the skies.

There was an Odin-ish character, but mostly there were humans and a few giants. As well as a few half-giants.

Read the series in English. I believe this was the original language.

Cannot remember covers. Cover art never has really been able to stick to me.

Seemed for around 16+, there was some nudity and mention of sexual acts directly, but not quite full on "Game of Thrones" describing the acts.

Answer 1

Well, I spent the last hour or two digging around some more, and eventually dug up the series I had wanted! For some reason I thought the series was not complete, although it appears now that 13 years later, there is nothing else in the series. Thanks everyone who responded.

The Wizard Knight by Gene Wolfe

The Wizard Knight is a series of epistolary novels written by fantasy and science fiction author Gene Wolfe. It chronicles the journey of Able of the High Heart, an American boy transported to a magical world and supernaturally aged to adulthood. Able (which is not his real name, but rather the name given to him) becomes a knight, and because of his connection with the magical and spiritual elements of the world around him is soon also dubbed a wizard.

....

The setting of the novel features elements from Norse, as well as Christian mythology and a smattering of European sources, such as Arthurian myth, and involves a seven-tiered world that is separate, but not completely detached from ours.

The kingdom that Able is taken to is called Celidon, and lies in the middle world, Mythgarthr. The world above it, Skai, is the domain of the Overcyns, who are roughly analogous to the Æsir of Norse mythology, and to whom the inhabitants of Mythgarthr properly owe fealty. Terrible giants called "The Giants of Winter and Old Night", also dwell there, and are their foes. Above Skai is Kleos, which, being far from Mythgarthr, is not much explained. Two of its inhabitants are introduced: Parka, a being much like one of the Norns, and Michael, who is much like the archangel Michael. The Overcyns of Skai owe their obedience to those who dwell in Kleos. The highest world is Elysion, and only The Most High God lives there.

Below Mythgarthr is the world Aelfrice, which is primarily peopled by small elemental beings called the Aelf. They belong to a number of clans such as the Fire Aelf and the Moss Aelf, and were themselves created by the collective creature Kulili, who lives in Aelfrice still. The Aelf properly owe their worship to the people of Mythgarthr, who they call the "old gods", but often stray in their faithfulness, sometimes even tricking humans into worshiping them. Below Aelfrice is Muspel, a world of fire and dragons. Last and lowest is Niflheim, the world of The Most Low God.

Time flows more rapidly the higher one ascends in the worlds; Able spends short times in Aelfrice and Muspel and returns to Mythgarthr to discover weeks or months have passed; similarly, he spends decades in Skai and returns to find only a week has passed below. It is implied that time stops completely in Niflheim and is infinitely fast in Elysion.

Answer 2

Sounds similar to The World of Tiers series by Philip Jose Farmer.

World of Tiers