From the amazon.co.uk home page, a general search on "treehouse" brings up some fantastic books in this area.
Some larger bookshops like Waterstones have copies to nosey through.
A Google UK search on "tree+house+planning" would be good to do as there have been some interesting issues in this area over the last couple of years...
A bit of background research with respect to building regs and fire regs would be advisable if you are intending it for accommodation of others.
(Access by stairs or ladder for example usually requires a particular pitch angle and tread depth & height)
The Local Authority for the area you intend to construct this would be good for an informal approach to gauge their attitude to any application.
I would also look up how you would insure the structure for it's intended use.