In general, the first number of the version number represents a major revision. The second number is used for a revision containing several changes or new information.
Apple recommends that you use version 1.0 for the first asset delivered to Apple Books. The version number is consumer-facing and is meant to manage delivering updates to consumers. Increment future versions as appropriate.
Each time you update a book, at least the right-most segment of the version must change.
For example, if the first version of the book was 1.0:
A revision could be 1.1
A total rewrite could be 2.0
When providing version numbers:
Previously-published books that used version numbers are required to continue to use version numbers. Enter a new version number when publishing an update.
Use dot-separated integers, with a maximum of one dot, two components, and three integers per component. For example: 111.111
1.10 is considered larger than 1.9
Leading zeros are ignored. 1.01 is read as 1.1
Letters are not permitted.