I could only read books in iBook format if i buy from the store?

[Trance]

for example is there a type of format i could convert ereader books so I could read them in iBooks (Not Pdf)?

 

[CylonGlitch]

There is a software package called "Calibre" that will convert the books to/from just about any unprotected format.

Convert your PDF files to ePub if you want too, although iBooks does a good job reading PDF files as well.

If you BUY the books from the iBookstore, they will, likely, have DRM on them, it is possible to remove this DRM but you'll have to google that.