I don't closely follow the Sony Portable Reader forums, and I had never heard of the
eBook Library Software as a stand-alone product. However, it is indeed advertised as a Desktop PC solution, with optional PRS-50X.
All Windows-based ebook reading software is free. Unless you expect to buy a PRS-505 portable reader, your best Windows options are MobiPocket, or MS Reader, or perhaps eReader. The MS LIT format is the best of the older generation of formats, but MobiPocket's MOBI/PC format is the most widely supported on E-Ink devices and probably has the most titles.
The advantage of Adobe Digital Editions is that it supports the best format of all, ePub. However, ePub is new and there are relatively few .epub ebooks available. If you go with ePub, then for a while you will be buying or downloading LIT ebooks and converting them to ePub (very easy to do).