A former District police officer whose undercover adventures once involved getting chummy with drug dealers, gamblers and gunrunners has a new undercover role: a knight in shining armor.

In his free time, Edward J. Jagen, 40, portrays Sir Edward, the central character in a book he wrote called "The Good Knight Story," a fairy tale designed to teach children how to avoid being abducted.

Some schoolchildren in Prince George's County read the book in class last year. Recently, the 6-foot-4 Jagen, clad in a suit of blue armor that he made to look like Sir Edward's, visited the school to present certificates making them Junior Blue Knights, each with a mission to teach "all children how to stay safe."

In all, Jagen knighted 55 students at University Park Elementary School in Hyattsville.

But Jagen is a man with a mission. He would like to see his book adopted for classrooms in other jurisdictions, including the District. The Prince George's County school system, the first to use the book, has approved it for use in elementary school counseling programs in the fall, said Louise F. Waynant, associate superintendent for instruction.

"We don't want to frighten children but make them aware of a very sensitive subject," Waynant said. "One of the strong features of the book is that it creates awareness without creating fear."

Using a Middle Ages setting, the book tells children of tricks that might be used to lure them away from the safety of their homes, schools or neighborhoods. In one case, the story's villain persuades a girl to follow him into the woods by telling her that her mother has sent him to bring her home.

To capture the children's attention, Jagen has woven his story around an imaginary kingdom called Eagleton, described in his book as "within a land one step back and three to the side of where you live now." In Eagleton, the king and queen hold festivals at their castle each season to honor children born in that season. All is well until the children born one spring suddenly begin to disappear.

When Sir Edward, a retired knight who becomes the story's "Good Knight," is asked to find the missing children, the story becomes laced with enchanting touches, including angels who grant four wishes to Sir Edward, a magical mouse named Mac Aroni, and a sword named Tenacity that represents "the never-ending quest in pursuit of the truth." But inside the kingdom also lurk a few untrustworthy residents, including a hungry giant, a dragon and the Bony Dead.

One by one, Sir Edward rescues the missing children, after which they describe the tricks a stranger used to lure them away. The stranger invariably turns out to be someone who had been well respected in the kingdom.

The revelation is intentional.

"Parents tell their children to watch out for strangers and then say that the stranger is someone you don't know," Jagen said. "Children need to know that the stranger who could hurt them may be someone they have grown to trust."

Jagen said he learned that lesson as a child; he was abused for several years by a family maid.

"I saw child abuse while I was growing up," Jagen said. "It eventually stopped in my life, but I wish I had told somebody about the abuse I saw in my buddies' lives. Because I said nothing, they suffered the consequences."

In a recent report, the Justice Department estimated that as many as 4,600 children nationwide were abducted by non-family members in 1988. Many of those abductions ended within hours, often after sexual assaults, but between 200 and 300 children disappeared for longer or were killed, according to the report.

Since last July, when Rosie Gordon, a 10-year-old from Fairfax County, was abducted and killed, there have been at least seven children abducted in the Washington area and more police reports of attempted abductions. Melissa Brannen, a 5-year-old abducted from her Fairfax County apartment complex during a Christmas party, has never been found.

To address abduction in a fairy tale, Jagen said, he merely added a touch of fantasy to events and people from his life and law enforcement experiences, which included investigating cases of missing and abused children. Eagleton, he said, represents the District, and the "tricks" used to abduct children have surfaced in actual cases of missing children.

Nevertheless, Jagen acknowledges that the book is a far cry from most of his law enforcement experiences.

Shortly after he joined the D.C. police, Jagen, who was assigned to the 6th District, became an undercover officer whose assignments ranged from infiltrating antiwar demonstrations in the early 1970s to breaking up drug rings.

During his 15 years on the force, Jagen said, he was considered a bit of a rogue. When his supervisors wanted him to confine his undercover work to an organized gambling operation, Jagen also managed to investigate a scheme to run guns to the Irish Republican Army. The investigation led to the convictions of four men.

Throughout his undercover work, Jagen pretended to be a Washington artist. But he doesn't pretend to have talent. In fact, to create enough artwork for an exhibit, he once splashed paint on a canvas and placed it in front of a fan. Some of the paintings created from that exercise sold well in this area, he said, chuckling.

Today, Jagen and others -- including Dennis V.N. McCarthy, a former Secret Service agent who subdued John Hinckley during the attempted assassination of President Reagan in 1981 -- operate Two Eagles International Inc., a security firm that specializes in services such as detecting industrial espionage and recruiting security personnel.

Despite his many successful cases, Jagen said he is haunted by a desire to do more to protect neglected and abused children. He recalled that one of his early cases involved removing an abused child from his home. Shortly thereafter, the child was returned to his family, and several weeks later Jagen had to remove the child's body from the home.

"The kids are the ones who are vulnerable," he said, "and I want my book to make them the teachers so that they can help each other see the danger."

Jagen's book cannot be purchased in bookstores, but it is being used nationwide as part of a child abuse prevention campaign by Childhelp USA, a national, nonprofit organization for the treatment and prevention of child abuse.

And Jagen said he is thrilled about letters he receives from schoolchildren who have read the book. Addressing their letters to "Sir Good Knight," some students from Hyattsville wrote that the book made them think of Melissa Brannen.

"I am impressed with the story you wrote and it can teach a lot of little and big kids," one fifth-grader wrote. "One time my friend and I were walking from this store . . . a white van stopped and the man said, 'Girls, where are you going?' My friend and I ran."