I've run across a thread on amazon's forums that contains search parameters for four traditional publishers who are making selected books available on kindle unlimited.
For US readers, if you have an amazon account and haven't activated your free trial, browse these searches to decide if it is worth it to you to take advantage of KU.
There are some really excellent books available for KU. I'll mention a few by name after the links:
Open Road Media has nearly their entire catalog available for KU readers.
Sourcebooks has made books available.
Pegasus Books seems to specialize in mysteries;
Houghton Mifflin has 180 books in their catalog available
Selected books by the following authors are included:
Open Road Media:
Pearl S. Buck
If you've got any recommendations of KU books, mention them in the comments (obviously, so long as you've read them, and you didn't write them).