Mandukya Upnishad, the shortest of all the Upnishads and a part of Atharvaveda, comprises merely of 12 mantrās but is the most profound one.
You read it and there is no necessity to read else.

The Mandukya Karikā, a commentary and collection of verses written by Gaudapāda Aćārya, the great guru of Adi Shankaracharyā, is the best now available text on it along with Shankara bhasya.

The whole 12 mantras deal with Om, the solemn reality, and try to reconcile the human brain between "this truth" and "that truth!"

The first verse/mantrā introduces us to Om.
Hari Om. The entire universe is the syllable Om. 
Everything in the past, present, and future is verily Om. 
That which is beyond time, space, and causation is also Om.

The second one leads to Brahman, the supreme.

All this is, indeed, Brahman. This Self (Atman) is Brahman. This same Atman has four quarters. (II)

The waking state, the state of dream, the dreamless sleep and the state of super conciousness (turiya).

Turiya, basically means fourth one.

Tye third mantra deals with first state.

The first quarter is called Vaisvanara, whose sphere of activity is the waking state, who is conscious of external objects, who has seven limbs and nineteen mouths and who is the experiencer of gross objects. (III)

(Where limbs and mouth aren't exactly literal)

The fourth one deals with second aspect of Atman, the Self in the Dreaming state, Taijasa. In this second state, consciousness is turned towards the inner world. It also operates through seven instruments and nineteen channels, which engage the subtle objects of the mental realm.

The fifth we discuss deep sleep. The person whom all experiences become unified, who is, verily, a mass of consciousness, who is full of bliss and experiences bliss and who is the door leading to the knowledge of dreaming and waking.

The sixth again discusses Brahman.
The one who experiences all of these states of consciousness is the omniscient, indwelling source and director of all. This one is the womb out of which all of the other emerges. All things originate from and dissolve back into this source.

The seventh again comes to "Turiya" the fourth(just a numbering order).
The state unperceived, unrelated, incomprehensible, uninferable, unthinkable and indescribable.
The state of Atman to be achieved.

Eighth one describes AUM and it's relation with Turiya.
A,U,M describe the three states of wake, dream and deep sleep and the silence that prevails behind these three states.

The 9th, 10th and 11th mantra dsal with the three states in detail with each syllabi leading to a state and how one can reach the supreme or superior state in each one.

The 12th and last mantra deals with the "silence!"
The fourth aspect is the soundless aspect of Om. It is not utterable and is not comprehended through the senses or by the mind. With the cessation of all phenomena, even of bliss, this soundless aspect becomes known

It is a state of nondual (advaita) reality—one without a second. This fourth state, Turiya, is the real Self or true Self. One with direct experience of this expands to Universal Consciousness.

The exact translation being:
That which has no parts (soundless), incomprehensible (with the aid of the senses), the cessation of all phenomena, all bliss and non-dual Aum, is the fourth and verily the same as the Ātman. He who knows this merges his self in the Self.

It will be good if you devote some time to this lecture by Swāmi Sarvapriyananda at IIT-K.

जगदम्ब!