Direct answer: Search the series name on Goodreads, which maintains a community-curated, numbered series list for the vast majority of published book series, including where prequels, novellas, and companion books fit relative to the main numbered entries.
Why series order isn't always obvious
Publication order and internal chronological order frequently diverge — some series are best read in the order they were written, others make more sense read in-universe chronologically, and author-recommended order sometimes differs from both.
Using Goodreads' series pages
Goodreads assigns a series number to each entry directly on its listing page, including short stories, novellas, and spin-offs, which is usually the fastest way to see the complete, correctly ordered lineup at a glance.
Checking the author's own guidance
Many authors explicitly address reading order on their official website or social media, particularly for series with a complicated publication history — this is the most authoritative source when it conflicts with fan-maintained lists.
Handling out-of-order or incomplete availability
If your library or preferred retailer doesn't have every entry, check whether the specific book you're missing is a self-contained companion novel (often skippable without losing plot continuity) versus a core numbered entry before deciding whether to substitute or wait.
Is publication order always the best way to read a series?
Not always — some authors specifically recommend chronological or author-suggested order instead, particularly for series with prequels written later.
Do companion novels count as part of the main series?
It depends on the series; some are essential to the main plot, others are optional side stories, and Goodreads' series pages usually note which is which.
Where can I find a series' full list if Goodreads doesn't have it?
The author's official website or publisher page is the next best source, especially for newer or smaller-press series.