Programming with API's

I have been looking lately play around with API's and try to get a general
feel for using them. I have a couple of questions:

1.  Are the system supplied api's useful?  Is there any real business need
to use these?

2.  Where can I get a better description of what an API does than just
looking at QSYSINC and QUSRTOOL?

3.  Where can I get API's other than IBM? Or are API's only supplied by
IBM?

4.  None of the examples in QUSRTOOL are in RPG IV, should they be in RPG
IV format and what do I have to do to get them in RPGIV format?

5.  And I am sure this has already been asked but.... Where can I get
information, examples, books, etc.... about how to use API's with RPG
IV.?(Not C )

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