This is the 62nd most frequent Latvian word.
vien
"Vien" in Latvian means "one."
Here, "vien" is part of "viena," the numeral meaning "one." It is describing the count of books the speaker possesses.
Man ir tikai viena grāmata.
I only have one book.
In this example, "vien" is used in the word "vienu," which is the accusative singular form of the numeral "one," referring to the friend accompanying.
Viņš nāca ar vienu draugu.
He came with one friend.
In this sentence, the word "vien" is part of the compound word "vienalga," which means "regardless" or "it does not matter." It signifies an indifference or resolve.