I just found this dutch Denksport puzzle book from 2011 in my drawer. I have no clue what this puzzle type is called, I just know that I used to like it and that it's called "Zoekplaatje" in dutch.
@michaln I think it's like Picross (or Nonogram, according to Wikipedia) but there the numbers are shown on the edges of the grid.
Edit: It could also be "Paint by pairs" or "Link-a-Pix" (found on the same Wikipedia page): "Paint by pairs or Link-a-Pix consists of a grid, with numbers filling some squares; pairs of numbers must be located correctly and connected with a line filling a total of squares equal to that number."
(https://en.wikipedia.org/wiki/Nonogram#Other_picture_logic_puzzles)