1. Your book describes Rx's `ObservableOnSubscribe<T>` as "for cases where we are only actually starting to obtain data for our stream when something subscribes to the Observable, not before." The Javadoc says it's "A functional interface that has a subscribe() method that receives an instance of an ObservableEmitter instance that allows pushing events in a cancellation-safe manner." I can't quite extract your definition from the Javadoc; it sounds like you're saying it's a "cold" Observable and the Javadoc doesn't mention that. Am I misreading the Javadoc? Does its definition entail yours? It makes me nervous that you are getting a meaning from it that I'm not - I must be misunderstanding something...
http://reactivex.io/RxJava/javadoc/io/reactivex/ObservableOnSubscribe.html

2. Also, according to that link, the `ObservableOnSubscribe`'s only method, `subscribe(ObservableEmitter<T> emitter)` is "called for each Observer that subscribes". ...so, the `ObservableEmitter` *is* the subscribed `Observer`, right? I think that's basically how it works, but `ObservableEmitter` is kind of a weird thing to call an `Observer`...

3. I'm trying to understand flatMap()'s inner workings. If I ha
...

query("Hello, world!")
    .flatMap(new Func1<List<String>, Observable<String>>() {
        @Override
        public Observable<String> call(List<String> urls) {
            return Observable.from(urls);
        }
    })
    .subscribe(url -> System.out.println(url));