A dream where one sees a holy person is not true; it is false just like any other dream. It differs in the aspect that 1. it is an indication of some purity of the dreamer's mind and 2. it may be indicative of a grace he is receiving.
A teaching received in the dream from a holy person, if not contradicting the general teaching of Dharma, can be taken further in the waking by the dreamer in his sadhana. A mantropadesha had in a dream is of this nature. If a teaching or advice received from a holy person or Ishwara in a dream is against Dharma, such a teaching is to be discarded and not to be pursued. Please read pages 57 and 58 of this book:
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There is a svapna shaastra which I have not seen though.
warm regards
subbu