Composer Kile Smith seems always to have been here - one reason, perhaps, why nobody saw his midlife creative breakthrough coming.

As curator of the huge musical lending library that is the Fleisher Collection, he held forth at the Free Library of Philadelphia's main branch for three decades, helping the likes of Charles Dutoit find obscure French repertoire and sending music on loan all over the world. His compositions turned up on contemporary-music concert programs, but not always high-profile ones. Suddenly in 2008 at age 51, Smith emerged as one of Philadelphia's more distinctive choral composers with his hour-long Epiphany-season Vespers, premiered - and subsequently recorded - by the Crossing choir and the Renaissance band Piffaro.

"Vespers is a great piece," said Donald Nally, founder and conductor of the Crossing. "It wasn't till we put the whole thing together . . . that we all went, 'Oh - I get this!' "

Four years later, the Crossing and Piffaro are reprising Vespers in a pair of Philadelphia concerts Saturday and Sunday before migrating to New York City on Monday, where Smith hasn't been heard to any great extent since he was composer-in-residence for the Jupiter Symphony in 1999. The notoriously self-effacing Smith, now 55, isn't exactly exhilarated.

"My first thought is that I'm going to get my butt kicked," he said. "I've gotten nice reviews. And I feel OK about the piece. The New York press knows Piffaro a little bit and they know the Crossing . . . but I do worry."

Stakes are higher these days. He retired from the Fleisher Collection in August, and though he has adjunct teaching assignments, he's now a full-time composer. Several pieces are in the pipeline, including an instrumental dance suite, The Nobility of Women, to be premiered by Mélomanie, the Delaware-based early-music group, on Jan. 14 at Wilmington's Grace Church and Jan. 15 at Presbyterian Church of Chestnut Hill. In effect, he's facing the pressures of a young composer, but with the realistic awareness of middle age.

That scenario shouldn't be as unusual as it is. Composers often don't write their most important works until after 40, when they have more important things to say as well as the means to get them clearly on paper. Vespers was a case of the right project at exactly the right time. Piffaro directors Joan Kimball and Robert Wiemken were old friends of Smith's and, having excavated music from the distant past for decades, wanted to commission something new. No stranger to writing sacred music, Smith is a longtime member of Holy Trinity Lutheran Church in Abington, where his wife, Jackie, is organist and music director. But his watershed moment came with his live encounter with the Monteverdi Vespers of 1610, a 90-minute piece containing every kind of music that was then imaginable.

"That really excited me. The idea of a choral piece that would take a whole evening was liberating to me," he said. "When Bob and Joan and I were talking . . . I had it in the back of my mind that this kind of thing would be possible."

Writing for Renaissance instruments wasn't exactly in the curriculum of Philadelphia Biblical University, where he was educated, but his daughter Priscilla was starting to play oboe with Piffaro, and was an in-house consultant.

"It's not like I found my voice. That voice has always been there," Smith recalls. "For some reason I felt confidence listening to that part of my voice and not caring whether it would be accepted as good or not."

The music feels effortlessly straightforward. The climactic Magnificat has a trio of three female voices in canonic counterpoint that sounds more casual than strict, as if the voices just happened to be singing the same melodies, staggered a few seconds apart. The choral writing is eclectic: Anglican harmonies are there, but so are influences from the 1950s pop harmony group the Hi-Lo's.

But putting it all together was a process full of false starts. Between the premiere and the recording, one movement was rewritten and another was expanded. The result was what Gramophone, the foremost English-language classical recording magazine, called "a spectacular work."

Since then, Smith has become one of the Crossing's pool of frequently commissioned composers, and has written the harmonically dazzling Where Flames a Word, which makes Vespers seem safe in comparison. Yet Nally's regard for the latter is undiminished: "There's a certain kind of freedom in Vespers that comes with the composer knowing he has a longer linear landscape over which to tell the story."

It's a landscape in which Smith lives. Growing up in Pennsauken, he had a religious-conversion experience that he describes as being enveloped with a feeling of warmth and well-being.

Now, people often observe how comfortable he seems in his own skin. As composers go, he's remarkably easy to be with, especially since his occasional use of street language tips you off that his beliefs don't exclude those who don't share them. His Fox Chase home - a well-kept Victorian - is like a self-contained cocoon. His daughters are home-schooled with help from outside tutors. There's a vegetable garden in the backyard. So steeped is the family in Lutheran tradition that its three cats have German names. And the male cardinals flying about in the backyard are called Fritz, the females Flicka. Allusions to St. Francis of Assisi, who reportedly talked to birds, are not unwelcome.

"I can see how that would happen," Smith says. "The birds get used to you!"

What emerges from this seemingly quirky household is impressive. Martina, 14, plays French horn, and Elena, 16, is principal cellist of the Philadelphia Sinfonia Chamber Orchestra. Priscilla, now 25, recently graduated from the Juilliard School, and with her father's social skills and work ethic, has become one of New York's busier freelance oboists. Smith jokes that he has to hire her in order to see her - or write a piece for her. That's the case with Nobility of Women: Priscilla is the featured soloist in a piece that draws its title from a 17th-century dance instruction manual.

Outside of the Crossing, though, the Philadelphia music community has been slow to commission major new Smith pieces. Partly, that's a condition of the 2008 economic crash, says Nally. Also, Smith is no pushover. Glory is optional but respect is not; he isn't one to give his talent away. But Nally believes that Smith's departure from Fleisher can only be good. "It's not just time to write but time to think," Nally said. "You can always fit in a few minutes at the end of the day for a few more measures, but can you find the time to think broadly about that when you're distracted by other things all day? I think it's great that he's taken the leap."

And who knows what New York will bring?

Information on Mélomanie:

302-764-6338 or www.melomanie.org

The "Vespers" concerts

are at 8 p.m. Saturday at Old St. Joseph's Church in Philadelphia, 4 p.m. Sunday at Presbyterian Church of Chestnut Hill, and 7:30 p.m. Monday at Park Avenue Christian Church in New York. Information: www.crossingchoir.com or www.piffaroplaza.com.

Contact music critic David Patrick Stearns at dstearns@phillynews.com.