This is the 508th most frequent Estonian word.
toonud
"Toonud" is the Estonian past participle of "tooma," meaning "brought."
In this sentence, 'toonud' is used as a past participle to indicate that the action of bringing (by the rain) has been completed in the past.
Eile oli vihm meid huvitavasse muuseumisse toonud.
Yesterday, the rain had brought us to an interesting museum.
In this case, 'toonud' functions as part of a subordinate clause referring to the completed action of bringing the cake.
Ta küsis, kes oli koogi toonud.
They asked who had brought the cake.
Here, 'toonud' is part of a present perfect question, indicating that the action of bringing the book back is being inquired about.