I'm tired of these old-school single-line statements and looked up one-line solutions in TypeScript. The following one is pretty clean in my eyes:
if (this.books.length === 0) this.userObservableService.getBooks();
// JavaScript single line 'if' statement
// Instead of
// if (this.books.length === 0) {
// this.userObservableService.getBooks()
// }