Line Spacing with the LaTeX Memoir Class: Why doesn’t setspace.sty work?
The LaTeX memoir class has its own way of setting the leading (line spacing) in a document. By default, it will ignore without comment any attempt you make to load in packages like setspace.sty or use commands like \linespread or \baselinestretch. Those people in all those other Google results for your problem, telling you that the answer is \doublespacing, \singlespacing? Not quite.
To invoke memoir’s native line spacing controls, use the Spacing environment:
blah, blah, blah
To use the setspace package, you need an extra line in your document preamble:
This was giving me a nasty headache, until I found this thread. There is probably more useful documentation of spacing commands in the memoir manual, but that is a huge friggin’ book; using the Spacing environment has solved my problem for now.