I would also recommend this book by Mipham Rinpoche on the Seven Line Prayer. The Seven Line Prayer is the essence of The Shower of Blessings, and the text is also is about the Shower of Blessings in that it contextualizes Mipham Rinpoche's deep understanding of the Seven Line Prayer and his revelation of the Shower of Blessings. It fortunately also contains a unique English translation of the essential Guru Yoga practice of SOB:
An Explanation of the Seven-Line Prayer to Guru Padmasambhava
by Jamgon Mipham
translated by Padmakara Translation Grouphttp://www.shambhala.com/white-lotus.html
I believe this Guru Yoga can be done anytime, day or night, and does not need to be accompanied with a Tsok offering, as evidenced by the practice here as it was originally revealed: as a concise Guru Yoga practice and not as a complete tsok sadhana.