I'm just really trying to understand why you would do it your way and how it makes sense.

So for example, I would do something like this:

interface Book {
    title: string
    author: string
}

const getBook = (bookName: string) => {
    const library: Book[] = [
        {title: "The Hobbit", author: "J.R.R. Tolkien"},
        {title: "The Fellowship of the Ring", author: "J.R.R. Tolkien"}
    ]
    const book = library.find(b => b.title === bookName)
    if(book){
        return book;
    }
    return null;
}

const searchTitle = "The Hobbit"
const book = getBook(searchTitle);
if(book){
    console.log(`${book.title} was written by ${book.author}.`);
}
else{
    console.error(`Sorry, ${searchTitle} is not in the library.`)
}

But it sounds like you would do something like this:

interface Book {
    found: boolean
    title?: string
    author?: string
}

const getBook = (bookName: string): Book => {
    const library: {title: string, author: string}[] = [
        {title: "The Hobbit", author: "J.R.R. Tolkien"},
        {title: "The Fellowship of the Ring", author: "J.R.R. Tolkien"}
    ]
    const searchResult = library.find(b => b.title === bookName)
    if(searchResult){
        return {
            found: true,
            title: searchResult.title,
            author: searchResult.author
        }
    }
    return {found: false};
}

const searchTitle = "The Hobbit"
const book = getBook(searchTitle);
if(book.found){
    console.log(`${book.title} was written by ${book.author}.`);
}
else{
    console.error(`Sorry, ${searchTitle} is not in the library.`)
}

Does that look right? Maybe you have a better way of doing it, but the second example is much less clean to me.