According to Slavoj Zizek in Ecology as a New Opium for the Masses, Varlam Shalamov‘s Kolyma tales is a good literature on Gulag, not “sentimental bullshit like Solzhenitsin”.

Text in russian is here. audiobook

there is also anthology of samizdat

by Varlom Shalamov:
What I’ve seen and learned at Kolyma camps

1.The extraordinary fragility of human nature, of civilization. A human turns into a beast in three weeks of hard work, cold, starvation and beating.

2.The main means of soul corruption is cold; people must have endured longer in the camps in Central Asia— it was warmer there.

3.I’ve learned that friendship, solidarity never arise in difficult, I mean really hard conditions — when life is at stake. Friendship arises in difficult but bearable conditions (in the hospital, not in the mine).

4.Learned that spite is the last human emotion to survive. Only spite lets the starving man’s flesh hold out — and he is indifferent to the rest.

5.Learned the difference between prison strengthening one’s nature, and work camp corrupting his soul. more>>