For solid, reliable advice you need to talk to a copyright lawyer or lawyers (at least one for each country involved).
That said, note:
- Being free and/or DRM-free has nothing to do with copyright. Allowing a work to be distributed free of charge in no way waves copyright nor does it grant any permission for further distribution unless expressly stated.
- Books, like many works, have multiple copyrights. The author's rights are for the words and the words only. Typography, artwork (cover or internal), binding treatment (if any), ... all have their own copyright, usually held by the publisher and/or artist. The words may be PD but the rest of the book may be copyrighted, possibly preventing any further/free distribution.