The mummy / adapted by David Levithan ; retold by Mike Dean. (OCLC #276480317)

A series can have different kinds of subseries: its parts can be named or numbered, and that distinction is indicated by which subfield is used for the parts. Named parts use ǂp, as in:

    ǂa 20 questions ǂp History

Numbered parts use ǂn, as in:

    ǂa Bitlet. ǂn Series 2

If the part has both a name and a number, both subfields might be used:

    ǂa British idealist studies. ǂn Series 3, ǂp Green

Catalogers may disagree on whether a phrase for part of the series is a name or a number. For example, the sets of low-vocabulary books I’m cataloging are in a main series called “Penguin readers” with subseries for the levels of vocabulary: EasyStarts, Level 1, Level 2, etc. I don’t think these really describe a sequence of parts (as mentioned in the docs for MARC field 830) but rather a descriptor of the reading level based on the number of headwords. Headings for these subseries have already been established in OCLC using ǂn for all except the lowest level:

    ǂa Penguin readers. ǂp EasyStarts
    ǂa Penguin readers. ǂn Level 1
    ǂa Penguin readers. ǂn Level 2
    ....

I am using the form that has been established to reduce future hassle in cataloging. Though it seems odd to me, it should not affect searching or display.