Atharva-Veda.

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The most important Upanishads are :
Isa, Kena, Katha, Prasna, Mundaka, Mandukya, Aitareya, Taittiriya, Chhandogya, Brihadaranyaka, Kaushitaki, and Svetasvatara and Maitrayani. These are supremely authoritative.

May the fundamental truths of the Vedas be revealed unto you all, like the Amalaka fruit in the palm of your hand. May Gayatri, the blessed Mother of the Vedas, impart to you the milk of Knowledge, the ancient wisdom of the Upanishads.

The Upa-Vedas
There are four Upa-Vedas or subsidiary Vedas:

1. The Ayurveda (science of life and health)
2. The Dhanurveda (science of war)
3. The Gandharva Veda (science of music)
4. The Arthasastra (science of polity)

The Vedangas:
There are six Angas or explanatory limbs, to the Vedas:

1. The Siksha of Maharshi Panini (Phonetics)
2. Vyakarana of Maharshi Panini (Sanskrit Grammar)
3. The Chhandas of Pingalacharya (Prosody metre)
4. The Nirukta of Yaska (Philosophy or etymology)
5. The Jyotisha of Garga (Astronomy and astrology)
6. The Kalpas (Srauta, Grihya, Dharma and Sulba); belonging to the authorship of various Rishis.

Siksha is the knowledge of phonetics. Siksha deals with pronunciation and accent. The text of the Vedas is arranged in various forms or Pathas. The Pada-patha gives each word its separate form. The Krama-patha connects the word in pairs.

Vyakarana is Sanskrit Grammar. Panini’s books are most famous. Without knowledge of Vyakarana, you cannot understand the Vedas.

Chhandas is metre dealing with prosody.

Nirukta is philosophy or etymology.

Jyotisha is astronomy and astrology. It deals with the movements of the heavenly bodies, planets, etc., and their influence on human affairs.

Kalpa is the method or ritual. The Srauta Sutras which explain the ritual of sacrifices belong to Kalpa. The Sulba, which treat of the measurements which are necessary for laying out the sacrificial area, also belong to Kalpa. The Grihya Sutras which concern domestic life, and the Dharma Sutras which deal with ethics, customs and laws, also belong to Kalpa.

The Pratishakhyas, Padapathas, Kramapathas, Upalekhas, Anukramanis, Daivatsamhitas, Parisishtas, Prayogas, Paddhatis, Karikas, Khilas, and Vyuhas are further elaborations in the rituals of the Kalpa Sutras.

Among the Kalpa Sutras, the Asvalayana, Sankhayana and the Sambhavya belong to the Rig-Veda. The Mashaka, Latyayana, Drahyayana, Gobhila and Khadira belong to the Sama-Veda. The Katyayana and Paraskara belong to the Sukla Yajur Veda. The Apastamba, Hiranyakesi, Bodhayana, Bharadvaja, Manava, Vaikhanasa and the Kathaka belong to the Krishna Yajur-Veda. The Vaitana and the Kaushika belong to the Atharva-Veda.