I still use my Sony 505 and the Sony Library Software without any problems. I'll try to answer some of your questions.
First, at the top of the Sony forum are sticky threads with information. The
Sony Reader Info, Start-Up & Reset Guide sticky thread is a good start as it contains links to MobileRead's
wiki.
The "
Installing the Sony Reader software" link gives a link to download the Sony Reader Library Software from SonyStyle.com if you have trouble installing.
The original thread, with the collection of information given by various MobileRead members, can be found in the Sony forum
here.
Second, I don't use my Reader to play mp3's, someone else may have a better idea about this.
Third, I use the Reader software and have never had any problems. I only keep about 200 books on my Reader and put them in collections I've made on the 505. My personal book folder system stays intact on my computer and is backed up to an external hard drive.
As Fayth mentioned, you can download
calibre, a great program for managing book collections, and Kovid, the creator, frequents the forums here. We even have a dedicated
Calibre forum. Also,
a FAQ for calibre is available.
Fourth, I don't know about using the screen savers, but you might want to check out our
Sony dev corner to see if anyone has any information about this yet.
I hope I was able to provide something that may be of help to you.