The first is a book called “Flexible Circuit Technology” written by Joe Fjelstad. It is available at www.FlexibleCircuitTechnology.com, or by clicking on the picture to the right.
Saturn PCB Design, Inc. has produced an electronic designers toolkit, with many helpful features built in, including impedance modeling.You can get your copy for free here.
The IPC has a guide to help you design rigid flex and multilayer flexible circuits. It is called IPC 2223 – Sectional Design Standard for Flexible Printed Boards. You can purchase a copy for download here.