There is only one official Koran (for Islam). A certain Caliph centuries ago decided which version was the correct one, and had all other ones destroyed. For the more serious reader, go on to read the Hadiths, a series of sayings attributed to Muhammed. As far as translations go, I don't have any comment. It is rather dry no matter what.
For Hinduism, there is a great variety of books to read, some of them are very entertaining. I recommend the abridged versions of the Mahabharata and Ramayana, as abridged and translated by the author Narayan. They are very short and readable. The non abridged versions run to several volumes, and some translations are tricky. This is to start with. Next, read the entire Bhagavad Gita, which is an exerpt from the Mahabharata, and contains the most 'religious' of the content of that epic book. This should be available anywhere. I do not recommend any of the translations from the Hare Krishnas, (AKA Krishna Conciousness Movement) as they are a modern sect, and if your goal is general knowledge, much of the commentary and 'explainations' they add are extraneous, and specific to their own beliefs, rather than reflective of how the book is viewed among mainstream Hindus. Any decent translation of the Bhagavad Gita is short and easily readable. For more advanced studies, you should read the Rig Veda and Upanishads. The Rig Veda is the oldest of the Hindu scriptures, and possibly the oldest scripture of a religion still practiced, so old that many of the gods mentioned are no longer worshipped, but it provides interesting context. As you read these books, you should reflect that they are the foundation, not a summary of Hinduism as practiced today, which is so diverse and manifold as to defy any attempt to summarize.
As far as the Chinese classics, these are widely and well translated for the most part. The Tao Te Ching is, in my opinion, the most accessible religious work ever written. Simple, beautiful and poetic. the writings of Confucious, the Analects and the Great Learning, are dry and more scholarly, but readily available. More advanced learners will want to read Mencius (Meng Tzu). As far as Chinese 'folk' religions, again they are too diverse to properly summarize, though all have been heavily influenced by Confucianism, Taoism, and Buddhism.
Buddhism is also difficult. In college, I read an anthology called 'Buddhist Mahayana Texts' which contained certain versions of the story of the Buddha's life. Next I read the Pali Canon, so called because that is the language in which it is written, which is a multi volume work which collects the Buddha's sermons. Since he travelled around preaching, many of the sermons are repeatetive. However, both of these are writings of the Mahayana tradition. The Therevada tradition has its own texts, which I have not read. For Zen Buddhism, there are various collections of the writings of various Zen masters, the most prominent of whom was Joshu. I don't have much guidance to offer here. Tibetan Buddhism is also challenging. The Tibetan Book of the Dead is probably the most canonical, but there are a huge variety of other writings which I am not qualified to judge.
If you are feeling ambitious, you might try the Zend Avesta, the holy book of Zorastrianism. This is not a major religion any more, but used to be, and heavily influenced several other religions. It is very dull reading, however.