I compare the books around a continuing game.
The useful next purchase depends on the rules your group uses, the campaign you want to run and the material already on your shelf. I put those relationships ahead of a longer shopping list.
How I approach a comparison
A supplement is useful for a particular job.
A starter set, a full rulebook and a campaign book can serve different stages of play. Their titles alone rarely explain what remains useful when your group moves on.
I organize publicly documented contents, edition requirements and purchase formats. These references do not claim playtesting, access to an entire book or an assessment of game balance.
Editorial habits
Keep the table and the edition in view.
Name the required rules
Identify the system and edition before discussing optional material.
Explain the additional job
Separate ready-made adventures, reference tools and player options instead of treating every book as another core rulebook.
Leave a clear boundary
Keep publisher claims, documented contents and unresolved requirements distinguishable.
A requirement or edition has changed?
Send the article link and the publisher statement that clarifies it. Specific corrections help the next group make a better choice.