A portmanteau is a British term for a leather suitcase--specifically, a suitcase that can open into two parts connected by a hinge. Frankenstein finds the portmanteau while he is performing some outdoor chores. Inside the suitcase he finds books and a series of articles which he begins to read. The books include Plutarch's Lives and Milton's Paradise Lost.
A portmanteau is a suitcase. The monster finds a suitcase in Chapter 15. In the suitcase are a number of books: Paradise Lost, Plutarch's Lives, and the Sorrows of Werter.