There is a revealing moment in Isabel Hardman's book where the author, a political journalist who lives with post-traumatic stress disorder, joins a forest therapy session. The therapist encourages her to "connect" with herself and "experience nature better". Hardman wanders through the wood and finds a small hornbeam, which is twisting up towards the light, struggling to make its way in the shade of a mature oak. She is attracted to its shape, admires its bark, and draws parallels with her own life: how long it takes to heal and grow, how the scars we gather can still be beautiful "like the zig-zagging trunk of this young tree". She reaches up and snaps one of its twigs: the tree is dead.

"Serves me right for being so dreadfully whimsical," Hardman writes. "There seemed to be no neat life lesson here, nothing you'd want to write on a fridge magnet or share on social media. I'd come here hoping to connect with myself, and instead I'd been drawn to a tree that was secretly dead."

It is a valuable lesson in Hardman's The Natural Health Service, a practical and self-aware account of the relief from mental illness to be found outside. Hardman, and the many people she meets, identify respite, recovery and resilience in walking, running, cold-water swimming, gardening, "forest bathing", birdwatching, botanising, horse riding and caring for pets. The common denominator is what Hardman calls "the great outdoors", that plangent, hearty Victorian-sounding cliche. But as she shows, other species and their ecosystems can be rebellious medics. At times, the "natural" world resembles the magic mirror that undercuts Snow White's stepmother: rather than reflecting back ourselves, it is alive with its own agency, a challenge to our narcissism.

The Natural Health Service is one of a rapidly growing forest of new books that examine cures found in nature. This winter alone has brought the publication of The Wild Remedy by Emma Mitchell; Losing Eden by Lucy Jones; Rootbound by Alice Vincent; and Wintering by Katherine May. One of last year's unexpectedly prominent books - unexpected because it was rejected by publishers and crowdfunded via Unbound - was Bird Therapy by Joe Harkness.