There are many books published about textile design but fewer focusing on heat transfer printing specifically. Two we have found helpful are featured below. Although neither of them is new they are both available from Amazon.

Designing and Printing Textiles - June Fish
Published by The Crowood Press
Packed with creative ideas and with a particularly strong section on working from paper onto fabric.
Published by The Crowood Press
Packed with creative ideas and with a particularly strong section on working from paper onto fabric.

Heat Transfer Techniques - Dawn Dupree
Published by A&C Black
Lots of techniques explained and an excellent start point for experimenting withembossing, flocking and fusing.
Published by A&C Black
Lots of techniques explained and an excellent start point for experimenting withembossing, flocking and fusing.