This plugin embeds an API documentation browser and search functionality directly into Geany's user interface.

Devhelp is an API documentation browser mainly aimed at GNOME-related libraries, although there are Devhelp books for a wide range of library APIs. Check your package manager and/or Google and you should have no trouble finding books that can be viewed with Devhelp.

Geany is a small and lightweight integrated development environment using the GTK+ toolkit.