I can't find this as a US ebook, but I'll nominate it anyway. I have a paperback and its been in my TBR list for a while now.
The Master and the Margarita by Mikhail Bulgakov.
Spoiler:
Nothing in the whole of literature compares with The Master and Margarita. Full of pungency and wit, this luminous work is Bulgakov's crowning achievement, skilfully blending magical and realistic elements, grotesque situations and major ethical concerns. Written during the darkest period of Stalin's repressive reign and a devastating satire of Soviet life, it combines two distinct yet interwoven parts, one set in contemporary Moscow, the other in ancient Jerusalem, each brimming with incident and with historical, imaginary, frightful and wonderful characters. Although completed in 1940, The Master and Margarita was not published until 1966 when the first section appeared in the monthly magazine Moskva. Russians everywhere responded enthusiastically to the novel's artistic and spiritual freedom and it was an immediate and enduring success. This new translation has been made from the complete and unabridged Russian text.