Luckily, I don't have to mess with that side of the biz anymore, Rainer. I just get books that've already been accepted for publication and tidy them up. The last time I actually worked in an office for a book publisher (Godine; after that I worked briefly for a chi-chi Italian art magazine called FMR--a most dispiriting experience)--was a long, long time ago. Back then, the company still looked at unsolicited manuscripts, but reading them was fobbed off on those lowest on the totem pole, usually some bright-eyed recent graduate from a fancy school whose trust fund permitted him/her to work for free. I did read most of the children's book manuscripts (I was children's book editor and managing editor), but luckily, they are short.
Even twenty years ago, the likelihood of an unsolicited manuscript being accepted was very tiny. Now, I wouldn't be surprised if Godine doesn't consider them at all. I know the big guys don't. Whenever some would-be writer asks me for advice on how to get published, the first thing I say is, "Get an agent."