When Peter Nelson was a boy, he was captivated by the treehouse illustrations in his copy of ''The Swiss Family Robinson'' children's book.
Now he has a book of his own about the romance of tree-borne structures - for adults as well as children. Mr. Nelson's book, ''Treehouses: The Art and Craft of Living Out on a Limb'' (Houghton Mifflin, 1994) helped get his contracting business literally off the ground.
Building treehouses and playhouses now keeps Nelson and two other workers busy most of the year. Aided by the publicity from the book, Nelson is working on treehouses in New Orleans, Chicago, and New York as well as here in the Pacific Northwest.
His enthusiasm for treehouses has to do with fulfilling dreams, not a love of heights, he says: ''I don't want to put anything higher than 15 feet.''
The book has helped make Nelson a nationally recognized evangelist for these whimsical structures. And he is not the only one who sees a bright future for putting people in trees.
Jonathan Fairoaks, a Philadelphia tree-care specialist, says treehouses help bring back a ''childlike essence.'' With the pressures of modern life, ''there is even more of a need for that type of fulfillment,'' he says.
Nelson's office in Fall City, Wash., is, of course, right out of a children's storybook. But until recently he lived on a treeless Seattle property.
''As adults, we need stairs. It's just as simple as that,'' he says, pointing up the winding staircase supported by posts cut from a tree that had to be taken down to make room for the office structure. (Some of his houses for kids have rope or wood ladders.) This staircase even accommodates Nelson's large dog, Bumper, a trusty companion who is half Newfoundland and half golden retriever.
Suspended between four trees, the office seems almost big enough for a weekend home. And on his desk are plans for just such a building - complete with kitchenette, bathroom, fold-down bed, and deck. He notes that costs are held down partly by the fact that treehouses don't need foundations.
''You can build this for 20 grand,'' he says of his office, which has a finished appearance inside.
For those who don't have that kind of budget, Nelson's book offers some tips on how to build one yourself. But it is mostly a picture book capturing the variety and fantasy of tree dwellings, from traditional Pacific Island structures to an Oregon bed-and-breakfast. His next book, he says, will go into greater detail on the how-to front.
Nelson's original business plan, beyond building and remodeling homes, was to make kids' playhouses, something he still does. These can be cheaper (such as $1,200) or in some cases more expensive than his treehouses.
AT one treehouse he recently built, Nelson finds third-grader Travis Gillmore and his friend Luke Henry-Sheppard eager to camp out overnight in their new arboreal abode. Travis's mom, Jo Ellen Gillmore, says that can't happen until the trapdoor is completed - it's now just a gaping hole in the bottom.
''It's just what I would have wanted,'' Mrs. Gillmore says of the structure as the children wave from the window of what they call their ''clubhouse.''
From his rural home not far from Seattle, Nelson also publishes a newsletter for treehouse lovers roughly twice a year.
One potentially knotty issue is building codes. Currently, treehouse construction is not governed by any specific codes or regulations, since they are temporary structures and have no foundation. But if more people start building them, especially large ones for adults, a demand for codes could follow, Nelson says. ''I feel I have some responsibility to spearhead this project,'' he says.
Mr. Fairoaks, who also builds treehouses as part of his business, points to the example of ultralight airplanes as a parallel case. The light, low-flying planes do not come under Federal Aviation Administration oversight.
''If we can keep our standards high and police our own industry, then I think we should be left alone,'' Fairoaks says.
Codes or no codes, ''treehouses aren't forever. I make it very clear'' to customers, Nelson says. A well-built one should last for years, but the tree will likely last longer, he says.
Unfortunately, Fairoaks says, most people who build treehouses don't understand trees.
A certified arborist, Fairoaks is working with Nelson to ensure that his forthcoming how-to book on treehouse construction teaches people how to avoid certain pitfalls that can kill or injure trees.
For example, many builders are tempted to put two bolts into a tree, one bolt just a few inches above the other. That will prevent sap from flowing properly in the space between the bolts, Fairoaks says. He recommends ''one good-size bolt'' in a particular area.
Another caution is not to prevent branches or the trunk from expanding outward in circumference as the tree develops, he says. ''Judicious pruning'' may also be needed to lighten the tree's load to compensate for the burden of the treehouse, he adds.
Fairoaks is working on a treehouse on his own property that will be 95 feet off the ground. He also makes rope swings mounted at similar heights.
''That's something else that brings the essence out of a person,'' he says.
Though many treehouse enthusiasts live in remote woodsy or rural settings, treehouses can also be in tightly built suburban settings, Nelson says.
* For Peter Nelson's playhouse catalogue, write: PJ Playhouse; P.O. Box 1136; Fall City, WA 98024. Phone: 800-248-8520.