This one is definitely harder of the secrets to describe, because it is the one I struggle with the most. I was snooping around when I came across one of these special areas in the nether. I consider this the easier (of the two I know of) to find due to it not needing any items.

Go to the nether and go on the weird walkway that is square shaped overviewing the whole thing.
On the left side, you will see a cave opening that blends in with the background. This is the entrance.

If you walk down, you will notice a very obvious lever. This controls a piston door.

This leads us to Saintes' (first?) Secret Room. It has a lectern with a hint alongside a decorative fire.

This provides us with our first hint to a secret place, alluded to be named the Cavern of Souls

The second secret room was found by the user AttackTeam

The entrance is this lava pool behind the nether building at spawn

Seems dangerous, right? Well the first step is to dive right in. Don't worry! It won't hurt ....

Dropping in flashes a sign with some coords. AttackTeam said it was a joke though.
[EDIT] I found said joke and it is just a sign from DystopiaBound

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Here is the view while you drop down, it may take a few minutes to get through all the cobwebs!

The next step to entering requires a few wind bursts and a few ender pearls.

Face south west and use a wind burst into the corner. Then throw your enderpearl quickly into the tunnel that opens up.

(Facing Front)
(Facing Back -- behind you where you came from)

You now are in another tunnel thing to navigate.

Use a couple more ender pearls to navigate this weird tunnel and you will come to a sign,

This however is a FAKE end, and you will need to use your wind bursts and ender pearls again to find the real one.

I don't have an image of this room because I find it particularly hard to get past this sign,
You need to use your windburst behind you at the stone block in the tunnel. This triggers a double piston and lets you inside the room.

But it leads to another lectern with a hint.

I am not aware of any more of Saintes Secret rooms, but there is a noteworthy correlation to the rooms. The fire from the first could relate to the lava pitt you use to enter, and the wind mentioned and used in this room could refer to the use of wind bursts to solve it. Potentially, this means the next room requires wind bursts to trigger?

Update!

AttackTeam found another book signed by Cain which mentions a cavern, possibly the alluded Cavern of Souls. He provided a map for a button that activates this secret room for me.

"there's one other book I found that might be related"

"button is somehwere on the yellow line"

The button you have to press is here

After you enter said tunnel, turn around and press this button

Then run to the other end, where you will see a new opening to pearl through

Finally you are inside

This is where we find the new book and hint that AttackTeam found, this leads me to believe Saintes Secret rooms might be connected to the music box secret rooms, but no evidence is cemented yet

NEW SLAINTES ROOM DISCOVERED (POSSIBLY?)

So i was looking for the secret mentioned in the miscellaneous thread by the tree and noticed a suspicious gap in the hill close by

going inside however uses a lot of enderpearls, as it is just as twisty and turny as the path to the room that was under the lava

once you get to a clearing you will see a lone lectern in a room covered in stone and andesite

while this may look like the end of the road, i believe there is more to it

i can get close to this little gap that you can possibly enderpearl ? through but i used two whole stacks and couldnt manage

if anyone can try let me know! it is on the upper right corner of the room (near the walls)