This is the 13235th most frequent Swedish word.
trettiotalet
"The thirties" (referring to the 1930s decade).
In this sentence, 'trettiotalet' refers to the decade of the 1930s as a period in history.
På trettiotalet flyttade många familjer till storstäderna.
In the thirties, many families moved to the big cities.
Here, 'trettiotalet' is used to express an approximate number, around thirty.
Antalet besökare var ungefär trettiotalet.
The number of visitors was approximately thirty.
'Trettiotalet' in this example specifies the decade from which the collection of books originates.