Play'n GO is a Swedish studio with one of the longest continuous release records in slots, and the second-largest collection here. It is unusual for how consistent its releases are: a Play'n GO game almost always uses a five-reel grid, a single clearly explained bonus feature, and no second screen.
Book of Dead is the studio's signature title and the game most responsible for the "book" genre — one symbol is chosen at random before the free spins begin, then expands to cover a whole reel whenever it lands. Fire Joker works the opposite way, stripping everything back to a three-reel fruit machine with a full-screen respin.
The Moon Princess and Rise of Olympus games are the studio's grid titles, where characters clear symbols from a cascading board and the round continues until no more clears are possible.
All Play'n GO games on this site load directly in the browser in free mode. Because the features are self-contained rather than layered, they are a good place to start if you want to understand how a modern slot bonus actually resolves.