From an Introductory Blog Post from the Biodiversity Heritage Library (BHL):
The development of a new API version was spurred by the recent introduction of full-text search to the BHL web site. In addition to the inclusion of full-text search, the entire API has been examined and updated. New methods have been added, existing methods have been modified, and many methods have been dropped entirely (or incorporated into other methods).
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Because of the extent of the changes, version 3 of the BHL API has a separate endpoint from version 2. Both versions live side-by-side with one another, and do not conflict. Therefore, any existing users of the BHL API should see no disruptions.
API v3 endpoint: https://www.biodiversitylibrary.org/api3
API v2 endpoint: https://www.biodiversitylibrary.org/api2/httpquery.ashx
Long-term, it is expected that version 2 of the API will be deprecated and removed, so current API users are urged to look at version 3 and to begin moving to it.
Learn More, Read the Complete Blog Post (What’s New, What’s Changed, etc.)