Gustav Mahler composed Das Lied von der Erde in the summer of 1908; the sketched short score was completed on 1 September of that year, and the orchestration followed throughout the winter months. The idea to integrate song and symphony had been present from his First Symphony onward; Das Lied is the crowning achievement of that integration.
Das Lied von der Erde was the first piece Mahler composed at his summer home in Toblach (AKA Dobbiaco, in South Tyrol, Italy). Despite the isolated and idyllic beauty of the location, it was still an uneasy place for Mahler to be. His entire reason for relocating was that his previous summer home, Maiernigg am Wšrthersee (Austria), had picked up too much emotional baggage the previous year. The summer of 1907 is often compared to the three hammer blows in Mahler's Sixth Symphony: first Mahler was pressured into resigning his position as head of the Vienna State Opera, then his older daughter Maria (age 6) died a slow death of scarlet fever and diptheria, and finally Gustav himself was diagnosed with a congenital and incurable heart defect.
And thus the mood in Toblach was predominantly one of resignation. Not entirely so, however. While Mahler knew his death was looming much sooner than later, he wasn't ready for it to be immediate. Mahler was superstitious about "The Curse of the Ninth Symphony," the fact that so many of the greatest composers and Mahler's own influences (Beethoven, Schubert, Bruckner) had died immediately after or during the composition of their Ninth Symphonies. Das Lied von der Erde was Mahler's ninth symphonic work, but he dodged the issue by giving it its current title and the subtitle "A Symphony." The music deals with issues of death and loneliness, but the departure at the end of the work is still hopeful and beautiful, not the despair of one expecting death in the next five minutes. Such darker resignation comes in later works, such as the piece actually titled "Ninth Symphony," which was Mahler's final completed symphonic work after all.
The texts in Das Lied come from Hans Bethge's compilation of ancient Chinese poetry, Die Chinesische Flšoete. A friend gave Mahler a copy of this book after his diagnosis in 1907, and this new reading material both calmed and inspired the composer. The texts are not true to the original in many ways: the German translation actually came from a French translation of the original Chinese. Even if the translations had been exact, though, Mahler surely would have made changes of his own to personalize the texts to his music.
The music of each movement is very different from the other five, yet still an undeniable whole. Mahler uses a huge orchestra, though seldom is the entire ensemble playing at once. He masterfully combines individual colors from the orchestral palette to create scenes ranging from a tavern to a tableau of classic Chinese imagery to an impression of his own secluded landscape in spring. The use of alto and tenor as vocal soloists, voices which have similar range but entirely different tone quality, enhances the depth of Mahler's color choices.
Gustav Mahler never heard Das Lied von der Erde. Its premiere was presented by Bruno Walter and the MŸuenchner Philharmoniker in 1912, the year after Mahler's death. Mahler's method was to write a piece, rehearse it, edit it, and then have it performed. Thus, Das Lied von der Erde is an unedited look into Mahler's musical mind.

There exist a large number of recordings of this work. My single favorite is Bernard Haitink's recording with the Royal Concertgebouw. There exist several recordings under Bruno Walter's baton, all of which are wonderful as well, and of particular interest since Walter led the premiere.