They immersed themselves in the lives of 10 common trees - thus, the subtitle, Discover the Extraordinary Secrets of Everyday Trees.
"Because trees are big and essentially stationary, there is a tendency to view them almost like monuments - impressive but inanimate," she wrote.
Far from it. She and Llewellyn found they could barely keep up with all the activity - the unfurling leaves, the budding flowers, the early green pinecones.
"I am astounded by the genius of the thing," Hugo wrote.
I was astounded by how little of it I had ever noticed.
My favorite tree is a huge sycamore in my front yard. I look at it all the time, but somehow I had never seen the reddish stigmas on the female flowers - those little balls. They looked like something that should be on a coral reef instead of a tree.
It got me to thinking that, to be true citizens of the planet, shouldn't we be paying more attention to trees?
Maybe it's just wishful thinking, but there seems to be a lot of tree activity lately.
The Cape May Bird Observatory - bird being the operative word - now conducts workshops on trees. People are just curious about nature, said leader Mark Garland.
Prominent bird field-guide author David Sibley not long ago branched out - producing a tree field guide. "It could be argued that trees play a bigger role in our lives than any other living things," he said in an interview.
The Pennsylvania Horticultural Society's Tree Tenders program has been growing - 3,500 people have taken its nine-hour tree care course. (A new one begins Oct. 12.)
When the Morris Arboretum opened its Out on a Limb exhibit - with a 50-foot-high walkway out into the canopy of trees - attendance shot up by nearly 40 percent.
The staff conducted a survey. A key finding: People with young children simply wanted activities in nature.
"There's no video games out there or 3-D glasses or guns," said the arboretum's Bob Gutowski. "People are rediscovering something they have lost connection with."
Penn-sylvania, of course, has a long heritage of trees. New Jersey has its Pine Barrens. And what other American city can match Philadelphia's last mayoral election, when the candidates were all but in a bidding war to see who could promise to plant the most trees?
(Nutter promised 23,000. His 2009 Greenworks plan proposed planting 300,000 by 2015. Progress so far: 45,000. This year, the horticultural society announced a mission to plant a million trees - including the Greenworks ones - in the larger region. This fall, it's planting 17,000. Stay tuned.)
So I've been talking trees with a lot of people lately. Roll your eyes if you want, but I just love their passion.
Gutowski talks about crossing the Walnut Street bridge come fall and looking down to see "this great succotash" of color - the greens and yellows of the tulip poplar.
To him, noticing things about trees is a richer way of marking time. Seeing the seasons in the changes of trees "marks the progress in my own life," he said. "As your life passes, their life passes; stuff happens to both of us."
As you read this, trees are combating climate change. They also cool cities with their shade. They enhance the value of homes and businesses. They soak up stormwater and gulp down air pollution. They become books and homes.
Perhaps environmentalists should come up with their own political coalition.
Call it the Tree Party.
You can get to know trees just about anywhere. During their research, Hugo and Llewellyn met in the parking lot of a local fast food joint to exchange materials. Looking around, they realized they could have created the entire book just with the trees near the Burger King.
People share things about themselves when they talk about trees. A forester I barely knew told how he grew up in a religious family, but it wasn't until he spent time in forests that he discovered his own spirituality.
For Hugo, it's changed how she looks at the world.
She's seen a photo of a tree under which Civil War soldiers - many with amputated limbs - are sprawled. A horrific scene.
That tree is still alive, and she's visited it. In the presence of trees like that, she said, "you see the sweep of history differently."
And it's changed how she looks at herself. "When you are standing in the presence of this really old thing, you see how relatively fragile and short-lived you are."
Tree people say that autumn, when the trees are decked out in vivid colors, is a perfect time to wade in.
Tim Dugan, a state forester in the southeast region, expects this year to be a mixed bag. Because of the midsummer drought - no, the later downpours didn't fix things - some trees are already losing their leaves without first turning color.
But he expects others to put on quite a splash.
In an age of what we're calling "nature deficit disorder," naturalists and environmentalists sometimes differ on whether it's essential to learn the names of things.
Hugo worries that naming can be a trap, fostering the misconception that "by knowing a tree's name, we know all there is to know about it."
But she also loves the view of British author Will Cohu: "It is rude not to know the name of something that works so hard on your behalf."
For me, the name is the place to begin. Since the summer, I've been on a one-year campaign to learn to identify trees.
I knew what an oak was, generally. But now I know the white oak has rounded leaf tips and the red oak has pointy ones. And so on.
I'll need much more time to really learn about trees. How about the rest of my life?
Contact staff writer Sandy Bauers at 215-854-5147, email@example.com, or @sbauers on Twitter. Visit her blog at philly.com/greenspace