Natalie Wood's Death -- Marti Rulli's Book Triggered New Homicide Investigation

Natalie Wood's death has been reopened as a homicide investigation, primarily because of a book written by the captain of the boat where Natalie Wood spent her last minutes ... a book suggesting foul play ... this according to sources close to the investigation.

We've learned Marti Rulli -- who co-authored the book -- has been in regular touch with the L.A. County Sheriff's Dept. over the last few months. She has given Sheriff's investigators documents supporting material in her book, and we're told the Sheriff is so impressed he has assigned 2 full-time homicide detectives to the case.

In the book -- "Goodbye Natalie, Goodbye Splendour" -- Rulli and Splendour Captain Dennis Davern write about the night Natalie drowned. They say before Natalie disappeared from the boat, she was drinking and taking Quaaludes with her husband, Robert Wagner and actor Christopher Walken.

According to the book, Wagner became enraged when he saw Wood and Walken speaking, and smashed a wine bottle, yelling at Walken, "What do you want to do, f**k my wife? Is that what you want?"

At that point, Walken returned to his cabin and Natalie and Robert went to their state room. According to the Captain, he heard a loud argument between the couple and thumping sounds, and eventually silence.

A short time later, the Captain went to the deck and was told by Wagner, "Natalie is missing."

The book claims Wagner refused to let the Captain call the Coast Guard.

Wagner told the Capt. the dinghy was gone, along with Natalie, but some doubted that because she was deathly afraid of dark water.

The death was ruled an accident, but Rulli was like a pit bull since the book was written in 2008.

We're told the homicide investigators have already traveled to Catalina, as well as New Jersey to interview Rulli. They've also gone to Augustine, Florida to interview the Captain, and they're going to Hawaii to look at the Splendour.

7:25 PM PST A source familiar with the investigation tells TMZ the Sheriff's Dept. has not called anyone a suspect in Wood's death. It is a general homicide investigation for now, and will take "a long time to complete." The source adds, supporting documentation in several books about Wood's death -- including Marti Rulli's book -- triggered the investigation because the witnesses accounts of circumstances surrounding Wood's death did not square with what they told officials immediately after her death.

We're told no one from the Sheriff's Dept. has spoken with Wagner, nor has an interview been requested ... at least so far.

7:08 PM PST -- Robert Wagner's rep released a statement to TMZ ... saying, "Although no one in the Wagner family has heard from the LA County Sheriff’s department about this matter, they fully support the efforts of the LA County Sheriff’s Dept."

The statement continues, "[The Wagners] trust they will evaluate whether any new information relating to the death of Natalie Wood Wagner is valid, and that it comes from a credible source or sources other than those simply trying to profit from the 30 year anniversary of her tragic death."