Thank you for your interest in the game of sjoelen, you are right to think there is a more to sjoelen than to just throw a few pucks.
As far as we know there are no books out on sjoelen. Correction: there was a book published on sjoelen, but it is out of print.
We have met a person in the Netherlands who may be planning to write a book sometime in the future.
She is a world class player and we are in contact with her and we will post updates here.
I would advise you to watch the many videos on YouTube of players in the Netherlands and then practice a lot, hopefully several times a week.
What we did observe:
Most good players stand still when they play with their legs spread slightly to form an "A" and only move their upper body (arms and hands) keeping their focus on the slots during each round, but it was also advised to relax between each round and then refocus.
They practice with the first 5 pucks (which is legal) trying to hit the outside 2 slots (the 1 and the 2 slots) and once they finish the 5 they know what the board is like.
They then get all 30 pucks back to begin the game.
During the first throw or round they mainly focus on the inside slots (3 and 4) sometimes completely filling them and then in the later throws they fill up the outer slots. They don't aim to get them close they aim the get them straight through the slots. Very few remain in front of the slots. It looks like they never slam the pucks, but the pucks have enough speed to lightly touch the back of the sjoelbak, as if there were no portals.
I hope this helps you in your quest to become a better player, but above all: Have fun!