What to send with a Demo

[roughly]

Hi there,I'm starting to send Demo Reels to music libraries (after first asking for permission) and I was curious what I should be sending with them. I know what to send with an artist press kit, but I assume this is a bit different. Do I need to send a bio or press clippings? Also, should I make it formal with a cover letter or very casual with a handwritten note? Any advice of what has worked for you guys would be appreciated. Thanks!Theresa