After a hiatus of 10 years, Robert Boswell returns with a new novel — his seventh — which once again shines a searing spotlight on the human condition. "Tumbledown" follows the checkered fortunes of James Candler, a young, go-getting counselor at a mental health clinic outside San Diego. As ever with Boswell, secondary characters matter, and Candler's travails soon run alongside or intersect with those of his patients, friends and lovers. It is a busy and ambitious book but, by the same token, one that is bursting with life.

At the outset, Candler is on the up. He is poised to become director of the Onyx Springs Rehabilitation and Therapeutic Center. His sheltered workshop program in which patients (or for him, "clients") improve their concentration skills on an assembly line has been a hit. He is engaged to the stunning Lolly, who is soon to re-enter his life after a stint in London. However, after gently chipping away, Boswell allows cracks to appear. Candler has underestimated the severity of certain patients' illnesses. His best friend, Billy, has gone from hapless to hopeless. Perhaps worst of all, Candler has begun an affair with Lise ("pronouncing it like a rental agreement"), a girl who long ago enjoyed a counseling session with him and has ever since been "a little obsessed."

Candler is the novel's linchpin but Boswell ensures that supporting cast members pull their weight — their presence integral rather than merely decorative. Onyx Springs' ragtag bunch of staff and patients includes failed actresses and washed-up rock stars, a woman called Rainyday, a temptress named Karly and a man known as the War Vet even though he isn't one. Boswell, himself a former counselor, has drawn on his experience to superb effect. His "nimrods and dimwits" convince and entertain, but Boswell also calibrates the mordant comedy to offer witty asides on sex and relationships: "Love is a toxin," we learn. "It's released in the blood by the appendix, which is why thinking men have theirs removed."

Blended in with the mayhem are poignant observations and incisive commentaries on mental illness, particularly when patients refuse to take their medications and appear at their most vulnerable, and in the sections where Candler reflects on his artist-brother's suicide. Even our protagonist isn't entirely immune, being "a man with demons, who helped others by seeing himself in them." In the two weeks Candler and Lise have together before his fiancée's return, there are heartfelt meditations on perhaps that greatest mental impairment of all, the need to be loved.

If there is fault to be found it is in the occasional overdone dent in Boswell's otherwise durable prose ("homemade baklava, the crust as delicate as the eyelids of exotic birds"). But in the main this is classic Boswell, from the deft twining of pathos and hardheadedness to the casual references to his beloved Bruce Spring­steen and Tolstoy's "The Death of Ivan Ilyich," the latter of which Boswell dissected so well in his 2008 primer on fiction, "The Half-Known World."

"Tumbledown" is a welcome return from a grossly overlooked and underrated novelist. With luck the wait for Boswell's next offering won't be nearly so long.

Malcolm Forbes has written for the Times Literary Supplement, the Economist and the Daily Beast. Born in Edinburgh, he lives in Berlin.