How does one cite a book series/sequence?
The Purdue MLA site provides a recommendation for how to cite an entire television series, which is essentially identical to citing a unified work: instead of naming an episode, one names the series as a whole, and the date shifts from a single point to a range.

By analogy, then, to cite a book series, one should simply use the series name as the book title, and list the range of publication dates, e.g., 

@book{LOTR,
 author = {J.R.R. Tolkien},
 title = {The Lord of the Rings},
 year {1954--1956},
 ...