Word of the Day:

: a minor work (as of literature) — usually used in plural

The book is a collection of opuscula written by the author between her two major novels.

"[Maria] Artamonova offers short summaries of most of Tolkien's satellite opuscula in roughly their order of composition—The Father Christmas Letters, Roverandom, Mr. Bliss, Farmer Giles of Ham, "Leaf by Niggle," and Smith of Wootton Major." — Jason Fisher, Mythlore, 22 Sept. 2016

Opusculum—which is often used in its plural form opuscula—comes from Latin, where it serves as the diminutive form of the noun opus, meaning "work." In English, opus can refer to any literary or artistic work, though it often specifically refers to a musical piece. Being a diminutive of opus, opusculum logically refers to a short or minor work. Unlike its more famous relation, however, opusculum is most often used for literary works. The Latin plural of opus is opera, which gave us (via Italian) the word we know for a musical production consisting primarily of vocal pieces performed with orchestral accompaniment.