I used to think that high-end yarns like Colinette and Rowan were really expensive, but now I'm involved in the whole yarn making process, I realise the amount of effort that goes into some of these products.
I've had this book for a long time, but dug it out last night determined to immerse myself in some technical detail in honour of National Spinning Week.
The Yarn Book by Penny Walsh has just the right mix of history, technical detail (for yarn geeks like me) and technique. There's a fair bit about S and Z twist and their effects in plied yarn - something that I'm still not sure I've grasped completely. Lots of different types of fibres and techniques for making snarled yarns, over twisted, boucle effects, some colour effects in the carding process too. All in all a really good little reference book which I recommend.