It's difficult to write about Emmylou Harris without lapsing into a long train of superlatives -- she really does have one of the most beautiful voices of her generation, and her taste in material and skill in using her instrument is nearly faultless. However, as good as Harris is and as consistently strong as her body of work has been, one could make a convincing argument that she's been frequently underrated through much of her career -- more than just a lovely woman with a pure, clear voice and a fine ear, she's championed a number of gifted songwriters before they went on to have distinguished careers of their own (from Rodney Crowell to Gillian Welch), matured into a first-rate tunesmith herself, collaborated with a remarkable array of artists, and has never been afraid to take her talents into unexpected directions, from purist bluegrass to the experimental atmospherics of her work with Daniel Lanois. Songbird: Rare Tracks & Forgotten Gems is a hefty four-CD box set (with a bonus DVD) compiled by Harris in collaboration with James Austin that does justice to the scope of a career that's spanned five decades thus far, and unlike most multi-disc collections it isn't merely a super-sized "greatest-hits" collection. Harris and Austin have purposefully avoided her most recognizable work on Songbird, instead charting an alternate path through her back catalog. The first two discs of Songbird represent a roughly chronological overview of Harris' discography, beginning with an outtake from her little heard 1970 debut album, Gliding Bird (an album she's all but disowned in the past), rolling up to speed with two examples of her work with Gram Parsons, and then diving into her solo work from 1975's Pieces of the Sky to 2003's Stumble Into Grace, focusing on Harris' personal favorites rather than radio hits (which were in short supply from the 1990s on anyway). Discs three and four are devoted to collaborations and unreleased material, including several demos that have never before seen the light of day, highlights from her Trio albums with Dolly Parton and Linda Ronstadt, several tracks from the Gram Parsons tribute Return of the Grievous Angel, appearances on tribute collections and discs by other artists, and a handful of live tracks. It's telling that a private recording of Guy Clark's "Immigrant Eyes," recorded as a birthday present for Clark, is as carefully considered and beautifully rendered as anything on these four discs -- one thing that becomes clear is that Harris takes no shortcuts as an artist, and for the broad eclecticism of this set, she's uniformly intelligent, emotionally true, and thrillingly accomplished whenever she decides to sing a song. The set's packaging is handsome, and the accompanying hardbound book contains a brief but eloquent introduction from Harris, a fine biographical essay by Sylvie Simmons, and track-by-track notes by Peter Cooper (with plenty of input from Harris herself). The accompanying DVD, featuring nine television appearances by Harris, seems a bit slight by comparison, though it does contain a few prize moments, particularly a duet with Elvis Costello on "Love Hurts" and some raging guitar work from Buddy Miller on a performance of "I Ain't Living Long Like This." Songbird isn't the most concise celebration of Emmylou Harris' talents you can buy, but it's been assembled with a grace and sense of purpose worthy of its subject; it offers nearly five hours of superb music from a singular talent, and anyone who loves Harris' music will revel in it.