I bought a copy of the 6th Edition (pink title header where the Amazon image shows blue) as an update to the 1987 3rd Edition. The reorganised book remains "succinct, precise, clear an comprehensive". It is an invaluable guide to the beginner wanting to learn the elements of OS map reading and compass work in the uplands of Britain or someone wanting to brush up his or her route planning and navigation techniques.
The 6th Edition has deleted reference to the now historical 1st Series 1:50,000 OS maps and now includes a short section on GPS.
I advise the reader to make up his or her own mind where Peter Cliff offers strong opinions about certain topics, e.g., techniques for estimating time and the use of route cards.