The place to begin is vijnAnavAda school in India.
From what I've read, Gaudapada was, by tradition, "the philosophical grandfather of Shankara. His Karika on the Mandukya Upanishad is the oldest known systematic exposition of Advaita Vedanta. Gaudapada shows clear signs of familiarity with Buddhist philosophy, and both his language and his doctrine are close in many cases to Buddhist originals. This has led many scholars to speculate that Gaudapada himself was originally a Buddhist."
According to S. Vidyasankar, gauDapAda is the first historically known author in the advaita vedAnta tradition, whose work is still available to us, namely gauDapAdIya kArikAs, an expository text on the mANDUkya upanishad. The kArikA of gauDapAdIyacharya is the first systematic treatise on vedAnta. Notes Vidyasankar:
"...to the advaita school, all four prakaraNas are writings of a human author named gauDapAda, and are therefore not regarded as Sruti, even though the first prakaraNa is found interspersed with the sentences of the mANDUkya upanishad" and "...it is clear that the GK has been written in the context of a vedAntic dialogue with various schools of mahAyAna Buddhism, more prominently the yogAcAra and madhyamaka schools."
Other schools of buddhism such as vijnAnavAda and the madhyamaka school itself criticize yogAcAra for holding views that entail consciousness as an Absolute. gauDapAda apparently agrees with this evaluation of the vijnAnavAda school.
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'Roots of TM' is really the distillation of decades of research into the
teachings of Maharishi, Guru Dev, and the Shankaracharya tradition."
Another great book on TM from Paul Mason