Having never been a religious person myself (I previously considered the stories of creation to be nothing more than superstitious claptrap written and aceepted by gullible fools) I read this book with an open mind about the alternative explanations it puts forward. Sitchin is a very good writer, of that there is no doubt. His research is presented clearly and consisely without resorting to too much scientific jargon. He clearly has an excellent knowledge of astronomy and this ties together nicely with his theory about nibiru/the annunaki. What he proposes in this book seems to make an awful lot of sense, and you get the feeling that the creation story as told in the book of Genesis and others is actually trying to tell us more than just the tale of a divine creation that centuries of religious propaganda would have us believe.
However, the author would have us believe that he is the leading authority on translation of the ancient Sumerian scripts, but just a small amount of research will lead you to the conclusion that Mr Sitchin is in fact held in very low regard by his peers in this field. Most archaeologists and anthropologists who have studied the cuneiform tablets on which Sitchin's theories are based disagree with his translations of the ancient texts, and this is where his credibility falls apart because, as sensible as the whole idea seems, it all comes down to his own interpretation of the scripts. It would appear that his translations are based on his own pre-conceived ideas about the 12th planet, rather than his ideas about the 12th planet being based on academic research and accurate translation of the scripts.
Read it anyway - it's a very interesting book but, just as he asks the reader not to take the bible stories literally, don't take what he says as gospel either.