DownloadManager.java

public class DownloadManager implements Closeable {

    private static final int MAX_TRIES = 3;

    private final Realm realm = Realm.getDefaultInstance();

    @Override
    public void close() {
        realm.close();
    }

    public void addToQueue(Book book) {
        realm.beginTransaction();
        DownloadTask task = realm.createObject(DownloadTask.class);
        task.setBook(book);
        realm.commitTransaction();
    }

    public Iterable<DownloadTask> getQueued() {
        return realm.where(DownloadTask.class)
                .equalTo("status", QUEUED)
                .findAll();
    }

    public void notifyRunning(DownloadTask task) {
        realm.beginTransaction();
        task.setStatus(RUNNING);
        realm.commitTransaction();
    }

    public void notifyDone(DownloadTask task) {
        realm.beginTransaction();
        task.setStatus(DONE);

        // FIXME
        task.getBook().setDownloaded(true);
        realm.commitTransaction();
    }

    public void notifyFailed(DownloadTask task) {
        realm.beginTransaction();
        if (task.getTries() < MAX_TRIES) {
            task.incrementTries();
            task.setStatus(QUEUED);
        } else {
            task.setStatus(FAILED);
        }
        realm.commitTransaction();
    }
}

DownloadService.java

@Override
protected void onHandleIntent(@Nullable Intent intent) {
    DownloadManager downloadManager = new DownloadManager();

    for (DownloadTask task : downloadManager.getQueued()) {
        downloadManager.notifyRunning(task);
        Book book = task.getBook();
        try {
            downloadBook(book);
            downloadManager.notifyDone(task);
            Timber.w("Book successfully downloaded: %s", book.getId());
        } catch (Exception e) {
            downloadManager.notifyFailed(task);
            Timber.w(e, "Failed to download book: %s", book.getId());
        }
    }

    downloadManager.close();
}