I know exactly how you feel. I was in the same boat last year. What you
aiming to do is pretty straight-forward but I know that finding stuff can be
a bit of a headache.
There's no high level API as such - not like the Windows API - I guess it's
more COM rather than API. code  is written through CDO - I think the DLL is
PKMCDO. You can write ASP or VB code that will access your Sharepoint
objects, properties etc. Here's a good place to go:
http://www.sharepointcode.com/
Look in the CODE section.

Here's an example that deletes a document using ado going through the cdo-
it deletes a document (by Tom Thompson).

Dim objSPSDoc
Set objSPSDoc = CreateObject("CDO.KnowledgeDocument")

objSPSDoc.DataSource.Open
"http://fileserver/test/Documents/LinkChecker.txt", , 3

objSPSDoc.Delete

Set objSPSDoc = Nothing

-----------------------------------------------------

This one will move a doc. from Sharepoint to the physical file system, again
using ado. (Tom Thompson). You can use ADO to move the document the other
way too.

Dim objADOStream
Dim objCDOKnowledgeDocument

Set objCDOKnowledgeDocument = CreateObject("CDO.KnowledgeDocument")

objCDOKnowledgeDocument.DataSource.Open
"http://fileserver/test/documents/Jokes.txt", , 3, 0

Set objADOStream = objCDOKnowledgeDocument.OpenStream
objADOStream.Type = 1 ' adTypeBinary
objADOStream.SaveToFile "c:\test.txt"

--------------------------------------------------------

There's some good stuff on the Microsoft site  about file migration. I have
something similar at work. Let me know if you want it.