Do you underline or quotation marks when the book is more than 100 pages?

You use italics for book titles, regardless of their length.

Here's the breakdown:

* Books: *The Lord of the Rings*, *Pride and Prejudice*, *The Hitchhiker's Guide to the Galaxy*

* Short Stories, Essays, Articles, Chapters, Poems: "The Raven," "The Gift of the Magi," "The Tell-Tale Heart"

Exceptions:

* When writing about a book within another book: Use quotation marks. For example, "The Hobbit" is a beloved book within the larger *Lord of the Rings* universe.

* When writing about a book title within a book title: Use quotation marks. For example, "The Book of *The Book of the Dead*"

Remember, these are general guidelines, and specific style guides might have different rules. Always check the style guide you're following for the most accurate information.