Technical Engineering & Design Guides

Flexible Circuit Technology - Third Edition by Joseph Fjelstad

a excellent flex circuit design guide, flexis are not for the faint hearted or the poor, and flxi rigids are even more problematic.

A lot of design guides can be found on the Mentor Graphis site, quite often written by Happy Holden. They also have all the IPC-7351 guidlines (essential for SMD design) that use to be found at PCB Matrix.

The compliance club is a good source for EMC guidlines:
EMC Information Centre - The EMC Journal (Free in the UK)

And top of the list IPC.