Under cover and on the scene with Britain's SAS

BIG BOYS' RULES, by Mark Urban (Faber & Faber, $102).

MORE mumbo-jumbo has been written about the British SAS than almost any other military unit in history.

The regiment has taken on mystical proportions: a grouping of supermen, government-sanctioned assassins who tumble out of planes at Mt Everest altitudes, cross international borders on the wings of square parachutes, cross dark deserts at breakneck speedsand pop up anywhere. They appear in the South Pole (take a bow ex-SAS officer Sir Ranulph Fiennes), the North Pole, the middle of oceans, the roof of the world and, as Mark Urban points out in Big Boys' Rules, the indistinguishable battle lines and occasional foxholes of Northern Ireland.

The British Army's 400 or so SAS men (out of a total battle order of about 155,000 men and women) have garnered an enormous international reputation. The SAS' buzz words are not muscles and guts, although that is fait accompli, but more subliminal attributes like motivation, commitment and application.

Urban, a veteran military writer and war correspondent, reinforces the growing SAS legend by sub-titling the book ''the SAS and the Secret Struggle against the IRA''. In fact, Big Boys' Rules is a chronology of clandestine operations in Northern Ireland where the SAS is one of the many undercover units lined up against the likes of the IRA.

Urban has done a lot of research - on both sides of the political fence. Many of his sources were key participants in some of Ulster's most heinous and/or controversial ''incidents''.

As Urban points out, when he puts the SAS on the high pedestal he reserved for it, the regiment's presence in Northern Ireland is an elite within an elite. It is a reinforced troop of 20 men who have to pass a rigorous selection and preparation course atthe SAS Training Wing in Hereford for a one-year tour.