https://stackoverflow.com/q/70663447/1271826

FetchBook.swift

func fetchBook(id identifier: String) async throws -> GoogleBook {
    var components = URLComponents(string: "https://www.googleapis.com/books/v1/volumes")
    components?.queryItems = [URLQueryItem(name: "q", value: "{" + identifier + "}")]
    guard let url = components?.url else { throw URLError(.badURL) }

    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(GoogleBook.self, from: data)
}

URLComponents is a more robust way to percent escape parameters in a URL.

(By the way, the inclusion of the { and } is very unusual. I copied this from your code snippet, but generally you would just do

components?.queryItems = [URLQueryItem(name: "q", value: identifier)]

Yep, I noticed that as well. I read the documentation again and realized this call works better: https://www.googleapis.com/books/v1/volumes?q=isbn=identifier so I changed the queryitems to components?.queryItems = [URLQueryItem(name: "q", value: "isbn=\(identifier)")]

Or should it be as follows?

components?.queryItems = [URLQueryItem(name: "isbn", value: identifier)]

isbn=identifier is a value of q so I think components?.queryItems = [URLQueryItem(name: "q", value: "isbn=\(identifier)")] is correct

Yep, indeed. Very weird endpoint.