Series numbers

There are two kinds of series, numbered and unnumbered.

Numbered series

A numbered series is one where at least one of the TITLE records with that SERIES_CODE has a not null, not zero SERS_NUMB value. 

/domains/PRH.US/series/SOO

    "series": [
      {
        "seriesCode": "SOO",
        "seriesName": "A Song of Ice and Fire",
        "description": "<b>An immersive entertainment experience unlike any other...",
        "seriesCount": 6,
        "seriesDate": "2016-10-18",
        "isNumbered": true,
        "isKids": false,
        "seoFriendlyUrl": "\/series\/SOO\/a-song-of-ice-and-fire",
        "_links": [],
        "_embeds": null
      }
    ],

Series count

For a numbered series, the seriesCount property represents just the numbered books, not any additional books or boxed sets that also have that series code. In technical terms, this means it is the number of TITLE records that have a not null, not zero SERS_NUMB.

For example, the Song of Ice and Fire series, 'SOO', has a seriesCount=6 while having 11 distinct works with that series code.

/domains/PRH.US/series/SOO/works

        "recordCount": 11,

Sorting a numbered series

When you sort the works (or titles) of a numbered series using sort=seriesNumber, only the records with a series number are displayed. So for the SOO series, the total count is just 6.

/domains/PRH.US/series/SOO/works?sort=seriesNumber

{
  "status": "ok",
  "recordCount": 6,
  "startTimestamp": "2017-01-31T09:04Z",
  "endTimestamp": "2017-01-31T09:04Z",
  "timeTaken": 3,
  "data": {
    "works": [
      {
        "workId": 108336,
        "title": "A Game of Thrones",
        "author": "George R. R. Martin",
        "onsale": "1996-08-01",
        "language": "E",
        "seoFriendlyUrl": "\/books\/108336\/a-game-of-thrones-by-george-r-r-martin",
        "contribRoleCode": null,
        "contribRoleDesc": null,
        "seriesNumber": 1,
        "_embeds": null,
        "_links": [
          
        ]
      },
      {
        "workId": 548668,
        "title": "A Game of Thrones: The Illustrated Edition",
        "author": "George R. R. Martin",
        "onsale": "2016-10-18",
        "language": "E",
        "seoFriendlyUrl": "\/books\/548668\/a-game-of-thrones-the-illustrated-edition-by-george-r-r-martin",
        "contribRoleCode": null,
        "contribRoleDesc": null,
        "seriesNumber": 1,
        "_embeds": null,
        "_links": [
          
        ]
      },
      {
        "workId": 108331,
        "title": "A Clash of Kings",
        "author": "George R. R. Martin",
        "onsale": "1999-02-02",
        "language": "E",
        "seoFriendlyUrl": "\/books\/108331\/a-clash-of-kings-by-george-r-r-martin",
        "contribRoleCode": null,
        "contribRoleDesc": null,
        "seriesNumber": 2,
        "_embeds": null,
        "_links": [
          
        ]
      },

Unnumbered series

Unnumbered series are much simpler than numbered series. A series is unnumbered when none of its TITLE records have a not null SERS_NUMB value.

/domains/PRH.US/series/ETK

    "series": [
      {
        "seriesCode": "ETK",
        "seriesName": "eTicket",
        "description": null,
        "seriesCount": 13,
        "seriesDate": "2010-06-01",
        "isNumbered": false,
        "isKids": false,
        "seoFriendlyUrl": "\/series\/ETK\/eticket",
        "_embeds": null,
        "_links": []
      }
    ],

The seriesCount for an unnumbered series is equal to the count of works with that series code. You cannot sort the works (or titles) of an unnumbered series with sort=seriesNumber. If you do, you get back an empty list since it only applies to records with a series number.