Many will have read, or at least heard of, Jeff Jefford's seminal tome. Regulation price for the revised, updated Mark 2 edition from Grub Street is £40.00. However, PostScript Books have it on offer at £16.99 - a substantial saving.

This is an excellent book - highly recommended - and at this price is an absolute snip.

http://www.psbooks.co.uk/products/military-history/war-other/item/observers-and-navigators