Finding musical works

"Nesting boxes" or "Russian dolls"  is a term for considering different relationships that works by a composer have with each other. Most books aren't very interrelated with each other. There may be later editions, or translations. But the book generally stays the book; it doesn't hide within another category. Musical works, alas, are highly interrelated, in two dimensions. First let's look at the vertical dimension, from the top down.

Collected works: Everything a composer wrote
Collected [performance medium, e.g., piano] music: All of the piano music a composer wrote
Selected [medium] music : some of the piano music
Individual opus or publication
Work within the opus
Movement within the work (uncommon)

Contrast this to the way iTunes handles classical music. An album will contain a symphony in 4 movements. If you start at the top with the album, you can hear the whole piece. But the real organizing element is the track; if you start with the first movement and have your iPod set to "shuffle", you are almost certainly not going to hear the rest of the symphony. It's really a bottom-up organization.

"Selected" can be a little tricky. It can share elements of another medium box. For example, "Chamber music. Selections" may or may not contain a string quartet, but if it does, it will also contain chamber music which is NOT for string quartet. If it contained only string quartets, yes, those string quartets would be chamber music, but the publication as a whole would file under "Quartets, violins (2), viola, cello" (which used to be "Quartets, strings")

Using Beethoven as an example:
1. Works. "Works. 1961, Werke / Beethoven ; herausgegeben vom Beethoven-Archiv, Bonn, unter Leitung von Joseph Schmidt-Görg"
2. Collected string quartets: "Complete string quartets and Grosse Fuge from the Breitkopf and Härtel complete works edition"
3. A quartet opus: "Quartets, violins (2), viola, cello, no. 7-9, op. 59" (also known as the Razumovsky Quartets, because that's who Beethoven wrote them for.)
4. Individual quartet: "Quartets, violins (2), viola, cello, no. 7, op. 59, no. 1, F major. String quartet opus 59 no. 1 : first "Razumovsky" quartet, in F major / Beethoven ; introduction by Alan Tyson"

Horizontal relation: arrangements and derivative works.

In the literary world, there are translations into other languages. The whole point of a translation is to communicate as faithfully as possible the author's intent. Sometimes, this involves abandoning the literal text. For example, the Kabbalistic explication of an automotive drive train that appears in the English translation of Umberto Eco's Foucault's Pendulum is entirely different from the passage in the original Italian, because the puns and cultural references don't work in English. All translation involves some alteration. A derivative work is just that: a new work derived from the old. Sometimes the boundaries blur, and we have to consider intent. A Bible in faithful English would be a translation. A Bible in English in rhymed iambic pentameter would be a derivative work, because there would be another layer of creativity spread over (and overriding) the original.

Arrangements are the translations of the musical world. An arrangement is a musical work adapted for different instruments than the original; for example, an orchestra piece written for piano 4 hands. Sometimes this is done by the composer, but usually it's done by other people. The composer Percy Grainger was notorious for writing "dish-ups" (his term) of his music for other ensembles, to the point that librarians have given up trying to figure out what the "original" form is, and just use as a uniform title "British folk-music settings. $p Country gardens, $m band" or "British folk-music settings. $p Country gardens, $m orchestra" or "British folk-music settings. $p Country gardens, $m piano"

Most arrangements, however, are not so neatly labeled. What you will see in the catalog is "[Uniform title] ; arranged", and you'll need to look at the record to discover what it's arranged FOR. If the patron has an arranger in mind, it's much easier; just search for the arranger as author.