If it's the GEM Pocket Edition (i.e. the one that actually fits in a pocket) then it's definitely a good buy. Pretty much everyone on this forum has at least one copy - I have two, that I know of <img src="images/graemlins/smile.gif" alt="" />
The only thing I can think of that you may want to watch out for is the author's "taste tests" to determine if plants are poisonous. Other survival books I've read have stated that such tests should never be relied on - there are some poisonous plants (e.g. in the deadly nightshade family, I think) where even a small taste could prove deadly. Of course, if it's a choice between starving to death and poisoning yourself, you may have to pay your money and take your choice, so a "taste test" might prove useful as a last resort.
IIRC, the first aid advice (which I find is usually the weak point in survival manuals) is quite up-to-date and reasonable.
There are other, much larger versions of the SAS Survival book - the names are all so similar that I can't keep them straight - which doubtless contain much more useful information, but which don't fit in my shirt pocket; therefore, I wouldn't be likely to have them on me as I fled from the burning wreck. <img src="images/graemlins/frown.gif" alt="" />
The small size of the SAS Gem version ensures that I can have it on me at all times; the newer versions have end-flaps that you could use to hold some (flat) useful items, such as a Fresnel lens or a plastic bag with gin-soaked cotton wool <img src="images/graemlins/smile.gif" alt="" /> (j/k - vaseline would be better, but much less tasty <img src="images/graemlins/wink.gif" alt="" /> )
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"The mind is not a vessel to be filled but a fire to be kindled."
-Plutarch