The Haynes around here is called the Book of Lies (BoL) and that describes it fairly accurately Tim

Get one though as it'll give you a good overview but do accept it's not always the gospel truth and the BoL shies away from telling you the stuff you really want and need to know. It's all a bit superficial really although, as I say, it contains some useful stuff and is an essential companion to have.
All the BoL gaps are filled on here. A search of the forum will almost always come up with the answer you need and if not, it's an odds on bet someone on here knows the answer. Ask away as much as you need..
By "shop manuals" I guess you mean the pukka Citroen workshop manuals. These come in a multitude of thick binders (I have an incomplete set that stretches to seven binders) and they are not really a good DIY guide as they assume the reader to be a fully Citroen trained automotive engineer. They also go for quite a lot of money on eBay.
If you speak a little French the very best are the "Revue Technique" and these can be got hold of in France quite easily, I even saw one for the diesel Xantia in the big Carrefour in Calais last time I was there. They are excellent and they publish different ones for different models of Xantia rather than the BoL which tries and fails to cover all models in one book.
There are others and some are available as downloads (questionable legality).
A really great manual that covers (amongst other things) the hydraulics in great detail but in a very digestible way can be found
here It's a big PDF and well worth printing and enjoying as a bed time read.