Bar Harbor writer Kristen Britain, who works as a seasonal ranger in Acadia National Park, recently published “High King’s Tomb,” the third volume in her best-selling “Green Rider” series, and is at work on a fourth book.

“I think when you work for the government you learn a lot about chain of command,” she says.

“The structure of the park service; it’s kind of a big family. Even in trying times there’s an esprit de corps, and that’s kind of what I tried to get across in ‘Green Rider’ and its sequels.”

Classic high fantasy, the series revolves around members of a messenger service, the Green Riders, answerable only to the king of Sacoridia. Each has a special power, and each is called to the service; some, like Karigan G’ladheon, acquiesce to the call only reluctantly.

The third Green Rider book, “High King’s Tomb” (DAW Book Collectors, 2007, 688 pages, $25.95) came out early this past fall to positive reviews.

“It’s bold, full of wonderful new ideas, and more passionate — and funnier — than her previous books,” says Julie Czerneda, a writing friend of Britain’s and author of the popular Trade Pact Universe and Species Imperative series.

“I like it. I think it’s a fun book,” says Britain. “But it’s always a little nerve wracking before you see the final product.”

Its predecessors, “Green Rider” and “First Rider’s Call,” both had “terrific sales” in hardcover and paperback according to the publisher, and both were main selections of the Science Fiction Book Club.

“Green Rider” was Britain’s first published novel, but not the first she completed — that happened when she was 9 years old, growing up in the Finger Lakes region of New York state. It chronicled the undersea adventures of the author and her friends.

She had planned to be a cartoonist — even published a book of 13 horse cartoons when she was 13 — “but the ‘writing thing’ took over.” She got a degree in film production at Ithaca College in 1987, a logical prelude, she says with a grin, to joining the National Park Service.

She worked at six parks, including Acadia, as a seasonal ranger before signing on here full time in 1998.

“It felt the most like home,” she says.

Throughout her tenure as a full-time ranger, she was writing — both on and off the job.

“I roved the carriage roads on a bicycle, but then I kept on being handed writing assignments,” she says. “Just about everywhere you go in the park I’ve touched with my writing and editing.” That includes informational displays, brochures and material that ended up on the Web.

She tried her pen at short stories, but returned to her first love, novels.

“I think it was a process of having so many short story rejections that I went back to writing novels,” she says, again with a grin. Quiet and introverted, Britain has a sly, trapdoor humor that she often springs without warning.

She’s also thorough, and will take as much time as necessary to get her stories right — something her editor will attest to, she says. Cue another self-deprecating smile.

“She takes her time and she crafts her work very carefully,” says Czerneda, who shares an editor with Britain. “She’s not a fast writer. But she’s worth waiting for.”

“You’re creating a whole world,” says Britain, “not just landscape, but political systems, religion, culture.” She believes “a lot of fantasy suffers from a lack of authentic landscape.” And she didn’t want her book — or books, if she was lucky — to be that way.

“Most fantasy turns into series, but I was not going to assume,” she says.

Britain sold “Green Rider,” in 1996 (something she counts as one of the highlights of her writing career) but it didn’t hit the shelves until 1998. “First Rider’s Call” came out in 2003.

“Each book is different,” she says. “With the first book, I was totally relaxed. With the second book, not so much. Suddenly you get all these expectations on you and you’ve got to produce.”

Throw in some turmoil in her personal life, and it was a tough time for her.

She left her full-time job with the park service in 2004 and focused on herself and “High King’s Tomb.”

“I think I’m getting a better idea of how plot works; I’m a little bit more relaxed,” she says.

“We all learn by doing,” says Czerneda. “One of the most difficult challenges for any writer is finding the courage and confidence to write on a very personal level, to expose yourself through what you care about and how much you care. That makes for superior writing and I’ve seen it from Kristen with every new story.”

From a cozy (i.e., very small) cabin tucked into a copse off a gravel road in Bar Harbor, Britain writes in longhand on looseleaf notebook paper, then transposes everything into the computer herself, all part of her writing process.

She’s now at work on her series’ fourth volume, which continues the perilous journeys of Karigan G’ladheon.

Czerneda sees a bit of her friend in the Green Rider.

“The respect for nature — and horses. Kristen never shirks responsibility or hard work, another Karigan trait. And there’s something gently romantic about them both. She’s blushing now.”

Britain says she sees a similarity in that both she and her heroine are honest and naive.

“But I’m just a regular person. I’m not anything special.”

That’s her. Just your average everyday park ranger-turned successful fantasy writer.