Restrict these to their prescribed use, i.e. two or more works "contained in the physical item at the time of publishing, release, issue, or execution" (cf. MARC21 documentation, 501 field); for works bound together subsequent to publication, place this information in a 562 note with accompanying 977 fields. (see Section V.3.).
For groups of separately-published works gathered together for collective issue by a bookseller/publisher, with or without a title page (often referred to as nonce collections), use cataloger’s judgment to decide whether to create a single bibliographic record for the collection (preferably with a contents note), or to create in addition an analytical record for each item in the collection.