BOOK REVIEW : Journals of a Youthful Naturalist in Love : SUMMERS WITH JULIET <i> by Bill Roorbach</i> ; Houghton-Mifflin $19.95; 292 pages

Imagine Henry David Thoreau not only young and in love but as a thirtysomething contemporary, and you’ll have an idea of the delights of “Summers With Juliet.” Presented as a novel, the book is actually a highly polished, thoughtful journal of the eight holidays the author spent traveling around the United States and Canada with his bride-to-be, the winsome and strong-willed Juliet.

They meet on Martha’s Vineyard in 1982 when he’s a 29-year-old handyman with literary ambitions and she’s a 20-year-old undergraduate determined to establish an identity before marriage. A long-distance courtship ensues--Bill is working for a builder in New Hampshire; Juliet still has two years of college, one of which she spends in Paris. Their meetings during this period are tense and upsetting, fraught with the misery that comes when one lover is absolutely certain and the other still unsure.

“Juliet was in love with me, as she often said, but took pains to make it clear that she was not to be possessed.” Because the enforced separation makes Bill pensive, the reader gets to know him extremely well, sharing boyhood memories, participating in his adventures as a part-time plumber, carpenter, rock musician and apprentice writer; marking time until Juliet returns to New York. By then she’s sowed a few wild oats of her own and is willing to consider spending the entire summer with Bill.

The plan is to drive across Canada in a pickup, camping all the way; 15,000 miles or so, east and west, up and down, just the two of them, testing the relationship. By the time they’re approaching Victoria, they’re down to $4.29 a day each, and it looks as if they might definitely have a future, though each is privately dreaming “of our exquisitely separate apartments in New York, just two short weeks and a few thousand miles away.”

There’s no doubt whatever about the book’s staying qualities. That project is off to a glorious start, full of humor, information, passion, suspense and high spirits.

Writing about Mt. Saint Helens, Bill Roorbach hits full stride as a naturalist. He’d seen the volcano once before, in 1978, under somewhat unusual circumstances, but this view with Juliet is an experience of another order entirely. For one thing, the volcano has erupted and is no longer the user-friendly landmark it had seemed on his first visit, but is all menace and threat.

“Around a corner on the slickly black new road: the mountain. She was gray instead of white and green, seemed furious still, overlooking the destruction she’d made--her top gone, a tilted crater there, a pale plume of smoke rising.”

He takes Juliet’s hand and looks out over “the perfect, lovely devastation flecked with the green of new plants growing where their seeds had chanced to land.” Mt. Saint Helens is but one of Roorbach’s metaphors, some more far-fetched than others, but all marvelously realized and in the end, astonishingly appropriate.

There are segments on fishing that put Roorbach in the exalted literary company of writers like Izaak Walton and John Hersey; a chapter on turtles that succeeds, against all odds, in propelling the novel forward, and a piece on hummingbirds containing the amazing information that “a hummingbird’s territory is large or small in proportion to the concentration of good blossoms in an area and will always contain the same number of flowers,” regardless of square footage.

By the summer of 1989, when Roorbach takes us to a decrepit cabin on the River of Promise in the Montana wilderness for one of the most ambitious vacations of all, we’ve learned the choicest facts about giant sunfish, rattlesnakes, wild turkeys and the blue crabs that scuttle ‘round the ponds on Martha’s Vineyard.

By then we’re ready for “Visitors,” the hilarious chapter in which Roorbach describes the unforgettable local types who just happen by the place he and Juliet have rented, the humans providing experiences scarier and funnier than anything else in nature.

After that, there’s the wedding at the Roorbach family’s lake house in New Hampshire, in the presence of love “dark as the clouds that come from nowhere, as refreshing as the rain, a blessed, hulking, uninvited guest that the hired cop can’t see to bounce.” It’s been a long wait, but worth every second of the time.

Next: Carolyn See reviews “Love in Black and White” by Mark and Gail Mathabane (HarperCollins).