Hi Tom,
Not sure from your post whether you are wanting more info about the the practical techniques (fencing, footpaths, walling etc) or the rationale and planning behind habitat/countryside management work.
In either case the series of handbooks produced by BTCV are an excellent and accessible place to start. They contain information on the rationale behind practical conservation tasks and a detailed practical guide on how to undertake the jobs. They contain excellent illustrations too. The full list is here:
http://shop.tcv.org.uk/shop/category?l=level2;lid=59 but you could check if your library stocks them.
There is lots of info available on the web too. You will probably find that if you go to the website of any conservation organisation ( Woodland Trust, RSPB, Canals and Rivers Trust or Natural England) there will be sections relating to the principles of their work. wwwnaturenet.net is good too.
Hope that helps. If it's something more specific or different you're after let me know as i might have some more suggestions.
Happy Reading