This is the 7778th most frequent Japanese word.
吉永
"吉永" is a Japanese surname meaning "eternal good fortune."
Here, '吉永' indicates possession, showing that the book belongs to Yoshinaga.
吉永の本を読みました。
I read Yoshinaga's book.
In this sentence, '吉永' serves as a surname followed by 'さん', a respectful title used in Japanese language.
吉永さんは学生です。
Mr./Ms. Yoshinaga is a student.
In this instance, '吉永' is referred to as a name, used in a general sense to describe its rarity.