"The Mahavairocana Sutra" (BDK edition). This is the single most essential text because it outlines the Vehicle of Mantras at its inception in 7thC India. The Lotus Sutra Gandavyuha (Cleary trans.) The Prajnaparamita Sutra The Sutra of Fudo Myo-O Thr Pratyutpanna Samadhi Sutra (BDK ed.) The Summary of the Mahayana The Nikayas (Bhikkhu Bodhi trans.) The Amitabha Sutra
More important than the texts is to be in the lineage of the great teachers of India, China, and Japan who have transmitted it down to the present day. The Path has steadily evolved over 1400 years to its present highly refined state, and many of the specifics in the Mahavairocana Sutra have changed while the essence remains the same.