Minor change

Author: sypets

Documentation/NextSteps/Developers/Index.rst

@@ -18,8 +18,8 @@ in Extbase, the following resources are recommended:
 
 * book: `Michael Schams: "TYPO3 Extbase: Modern Extension
   Development for TYPO3 CMS with Extbase & Fluid"
-  <https://www.extbase-book.org/>`__ (3rd edition for TYPO3 9)
-* online resource: :ref:`t3extbasebook:start` (partly outdated)
+  <https://www.extbase-book.org/>`__ (3rd edition for TYPO3 9, not official documentation)
+* online resource: :ref:`t3extbasebook:start` (official documentation, but partly outdated)
 
 .. important::
    Note that currently, the online resource :ref:`t3extbasebook:start`