Musikalisches Opfer, BWV
AKA Musical Offering or L'Offrande musicale, this is one of the last (and best) works of J.S. Bach.
Bach was, during his lifetime, praised more as a scientist than a composer. His skills in acoustic were universally admired, and it was as an scientist in this field that he was elected to the swedish Academy of Sciences. He was often invited to try out newly build instruments, mainly organs for churches, and evaluate those.
In late 1740s, Frederic the Great (AKA Frederic II.) decided to buy some new cembalos for his personal use, or more exactly, for the use of his court band. The king himself played the flute, and he was pretty good at that, in fact he was a pupil of J.J. Quantz. He invited Bach to evaluate them. Bach dully arrived in spring of 1747 and got to work immediately, and the royal court and King himself were pretty impressed by his improvisations. Bach then offered to improvise on themes from the audience, as was the custom back then. Various themes were submitted and improvised upon by Bach, then, at the end, Frederick submitted the so-called thema regium, royal theme, extremely difficult to improvise upon due to it's highly chromatic nature. Needless to say, Bach did his job to the monarch's satisfaction.
Upon returning to Leipzig, Bach wrote a complete work upon this royal theme, and send an engraved copy to Frederic in autumn 1747, perhaps also hoping to get a job there.
The work itself consist of a variety of canons, including canon perpetuus; unlike Kunst der Fuge, formed almost exclusively of fugues, the canonic technique is predominant in Musikalisches Opfer. Two notable exceptions are the two ricercari. Ricercare is an older form of fugue, basically. There's one Ricercare a 3 and one Ricercare a 6, the numbers denoting the number of voices. A complete trio sonata is added to the Opfering as well, featuring the king's, instrument, the flute.
The work is one of the summits of the western music; it's often performed in a variety of ways. I personally recommend a Nikolaus Harnoncourt's version with Concentus musicus Wien available on Teldec 0630-13563-2.