Recently I learn this :
How to know all the possibility offered by elgg ?
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You can look at the info available at http://learn.elgg.org/en/2.0/ for some basic knowledge. For reference you can check out http://reference.elgg.org/. Apart from that it helps a lot especially when beginning developement on Elgg to look into the code of other plugins (bundled and 3rd party) to see how other devs are doing it. It might be a bit hard to understand how to use the Elgg API at the beginning when you are not yet familiar with it but once you have managed to understand some basics you will realize that it's not too complex as you will come across some typical functions and patterns again and again. Maybe best is to look for plugins that implement a similar functionality of what you are trying to achieve. Chances are high that you will find some useful code in such plugins that will help you to understand how it is done.