The Squatters Handbook has been published by the Advisory Service for Squatters in London since 1976. It is now in its thirteenth edition and provides 83 pages of detailed legal and practical information about squatting, homelessness and Travellers in England and Wales.
Be aware that the law is different in Scotland and Northern Ireland and that the info in the handbook won't apply there.
14th edition is updated with all of the new info that was available at the time of publication in 2016. I also have the 13th edition. The new one is well updated and is still very useful and packed with practical info. Recomended!