Xjyuk

Proper medieval artificial light sources rely only on fire, or on trapped fireflies.

Though it is not exactly a medieval technology, really dedicated alchemists (and a twist in the story) might come with the invention of the glowing sticks.

A glow stick is a self-contained, short-term light-source. It consists of a translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence, so it does not require an external energy source. The light cannot be turned off and can only be used once. Glow sticks are often used for recreation, but may also be relied upon for light during military, police, fire, or EMS operations. They are also used by military and police to mark ‘clear’ areas.

In terms of what was known before, say, the 18th century, there were very limited sources of what we'd normally consider rather dim light, but fire wasn't the absolutely only light source (not counting the sun, moon -- as it was thought to give its own light in pre-Renaissance times -- and stars).

Glowing insects were the commonest, and brightest. Fireflies can be nearly bright enough to (briefly) read by, if you can get even two or three of them near your page (their tendency to blink in synchronus is both helpful, because they're brighter, and unhelpful, because it'll be bright then completely dark). Firefly larvae are "glow worms" -- and I'm not certain whether they glow steadily or flash like fireflies.

Other sources of phosphorescence, like certain fungi (foxfire), are very faint -- easily bright enough to see in a dark forest, but not really bright enough to see by.

I can't say whether it was known in pre-scientific times or not, but the mineral sphalerite (zinc sulfide) is moderately common in some regions. It's the material that's incorported into "glow in the dark" paints and plastics -- it will absorb light energy and then release it over time, though it can't be "kept" and only lasts minutes to an hour or so unless there's a source of either UV radiation or something else (this was used in radium clock dial paint, excited by the radiation from the actual radium).

Pure phosphorus was made by alchemists well before the 18th century, and glows from slow oxidation even when stored in water -- but I'm not sure this wouldn't be a form of the lost fire.

Hammered iron.

https://www.youtube.com/watch?v=tXF60MOWUeY
Cold iron can be hammered until it is glowing red hot. That is a little bit of light. Also the hammering throws off sparks which also produce some light.

Not sure anyone is going to read a book with light like this but it might be a cool thing for a story - someone whaling on an iron rod until it glows then using it to light the way.

During daytime, they can use glass bottles with water, plugged in holes in roof as sort of light bulbs for home illumination.

https://www.instructables.com/id/How-to-build-a-SOLAR-BOTTLE-BULB/

Also with systems of mirrors and lenses its possible to illuminate, for example, cellars and mines using light of Sun.

Unfortunately, at night, you have to use candles, and fires.

Bioluminescent fungi aka foxfire. It's nowhere near bright enough to be a torchlight, but you may be able to have it cultured brighter. A specific one would the bitter oyster fungus. (Tastes nasty, apparently.)

There are a few natural sources of light, which could be harnessed if you really need to.

Lightning produce light. If lightning storms are something common in your setting, it's not unthinkable that someone (maybe with some help from magic) has learnt to harness such power to use as light source. Not sure realistic, but hey, it's a magical world...

Sparks. Technically fire (?), but not a real flame. Imagine how much sparks come from a wheeled wetstone when sharpening a knife. Now make it smaller and portable, with a smaller blade attached. Takes a bit of effort, but could be a source of dim light.

Bioluminescent animals and plants. From common fireflies, to rare fishes found on in the deepest trenches on the ocean floor, all the way to some kinds of plankton and algae or shrooms. All sources of dim light. Since it's a living being, it's going to requires some taking care of, or constant replacement.

Moonlights is obviously an option. Maybe your world has multiple moons, making full moons a much more common event.

Magic. I know you said u don't want magic sources, but what about "natural" sources that came to be thanks to magic? Maybe some crystal or other mineral (possibly not radioactive or toxic) that sheds some light? Maybe a glowing small animal, something hamster-sized or larger. Or maybe there's simply some glowing liquid (naturally, or by alchemical means). It's magic, doesn't need to have an evolutionary purpose or even sense!

One could heat a well-insulated mass of metal by using a solar mirror, and then use the black-body glow of the metal when the sun goes down. You could get yellow light at sunset, fading to a deep, soft red as the metal cooled.

The insulation would be layered, with porous pottery at the innermost layer and something more like horsehair on the outside. Insulation is important to reduce the losses, and help the metal heat faster and cool more slowly.

Magma emits a very beautiful light at night. Might be helpful if you don't mind living near a volcano. You may even scoop some lava from a nearby volcano and hold it in a ceramic or tugsten container, making your very own lava lamp (drumroll).

And while you may not have fire, if you scratch two iron rods really hard you still have sparks. One could crete contraptions which rub iron against iron and give off a little light as well.

Finally, since you are using the magic tag, you can take a page from Terry Pratchett's Discoworld's light dams:

An architectural and engineering feat in the Great Nef desert, designed and built by Goldeneyes Silverhand Dactylos.

While not described in the text, it would appear from context that these are otherwise conventional dams, strategically located in the Nef desert to trap and channel the sluggish Discworld light, so that it can be exported and/or sold on to other parts of the Disc that need it more than a desert does. The further details of the technology/technomancy involved have so far not been disclosed to us. The Light Dams and their devisor make an appearance in The Colour of Magic.

Well, it's certainly not historical, but it might be doable with tech that they could have used. A Mercury lamp.

If handed the requirements I can't think of any step that is impossible, though admittedly improbable and no reason for them to invent the tech themselves.

Glass tubing -- Difficult, but at least possible - would be very expensive in comparison to modern manufacturing techniques.

Mercury -- The knew have to produce this.

Electricity - Making batteries does not require advanced chemistry techniques.

You need high voltage. You could put enough batteries in series, but it will take a lot - you need about 250 volts DC to get a mercury lamp started that operates at 120 V.

There are still hard details. E.g., as the mercury heats up and converts to plasma, the resistance of the lamp drops, increasing current and heats in a positive feedback loops that leads to self-destruction. No modern circuitry to rescue you from this problem, but by integrating a variable resistance element in the circuit (hand controlled) until operating temperature is reached you could be OK as the resistance of the batteries in the circuit would provide some negative feedback of the current.

Mercury lights produce a lot of ultraviolet, which are normally filtered out (in one form or another) in lighting applications. You can go blind if you get too much. Can't think of a simple way to block UVA (glass blocks most UVB) using middle age tech off the top of my head.

Glow in the dark paint.

The brightest glow in the dark paint is supposed to be visible in the dark for 30 hours, which is plenty to get your peasants through the night.

A lamp like this would be clearly visible and would not require fire or electricity. Just charge it up during the day, and the photoluminescence should go all night.

According to Encyclopedia Britannica, glow-in-the-dark was first synthesized in 1603. It technically isn't medieval (5th-15th century), but it's still pretty close.