Singer/guitarist Kelly Joe Phelps is a curious performer. For his concert at Cambridge’s House of Blues a week ago Tuesday, he sat alone at center stage on a wooden chair, bathed in red and blue light, hunched over his guitar with the diligence of a cobbler obsessed with saving soles. He sang mostly about lost spirits — social outcasts like the gentle Tommy, who, fearful of a world he doesn’t understand, decides to leave it, and the hopeless, alcohol-stained Taylor John and his wife. They are among the characters populating his latest CD, the dark and emotionally vibrant Sky like a Broken Clock (Rykodisc), in which Phelps departs from his usual solo track to employ the rhythm section of Morphine drummer Billy Conway and Tom Waits/Canned Heat bassist Larry Taylor.

What’s curious about Phelps is that he has found so devoted an audience. (He sold out Passim last year and filled three-quarters of the House of Blues.) After all, his lyrics are riddle-like poems that require full attention. Not just because they’re bursting with small details. They are often linear stories, their choruses a few spare lines rarely sung or played exactly the same way twice. That’s because Phelps is a constant improviser, draping the chords that keep his numbers moving in a web of spidery single-note melodies, accents, and fills that fly spontaneously from his deft fingers or from under the slide bar he uses when he lays his guitar flat on his lap. He does much the same with his vocal melodies. His singing ranges from mumbling to a Waits-like growl to sweet tenor tones akin to the feathery moans of the great Delta bluesmen.

At the House of Blues, he looked as if he were in his own world as he played, eyes cast down, woolen cap pulled on tight. This wasn’t introversion so much as an effort to tap the stream where the waters of instant creativity and written craft mix. To get it right, he needs to put himself in a partial trance. Otherwise the slow, revelatory magic of numbers like "Capman Bootman," which unreels into an intimate self-portrait, won’t be nuanced enough to work. And if the audience isn’t absolutely focused, the spell is broken. During "Gold Tooth," a ballad about imbalance and loss, he stopped to tell a clutch of inconsiderate yappers to "shut the fuck up." That won applause from the faithful but seemed to rattle him. After all, high art of any kind is like walking a wire, and disruptions can be ruinous. But when an artist like Phelps is soaring above the fray, it’s a lovely thing.