To understand the Anzac spirit you must understand the spirit of Gallipoli.

The Anzac spirit forms the bedrock of the Australian and New Zealand national characters.  It was forged from a mateship which grew into something greater than the shared experiences of brothers-in-arms. 

The Gallipoli Campaign was a kind of crusade and a national rite of passage for three of the countries involved:  Turkey, Australia and New Zealand.  Each emerged from the devastating losses with an enhanced international reputation and its image clarified in its national consciousness.

Recent praise

The Spirit of Gallipoli is a little gem, mainly because of its clarity and Lindsay’s reliance of primary source material such as diaries and original photographs that bring freshness to his subject.
— The Newcastle Herald July 1 2006
Want to learn more about what happened at Gallipoli but haven’t found the time to wade through a historical tome? Lindsay’s engaging book could help. The former TV journalist turned historical author examines the momentous conflict in simple yet powerful terms, bringing to life the heroes and villains, the tragedy and the glory, while delving into the spirit that sustained the Anzacs as they fought so far away from home. Immensely readable.
— Sunday Telegraph/Herald-Sun Magazine April 22 2006
We’re all familiar with the story but often as not, we tend to get it in episodes. The value of this compact, straight-forward account is that you can get the full, bloody catastrophe in the one sitting. Patrick Lindsay, whose works include The Essence of Kokoda, not only uses the first-hand accounts of the soldiers to put you on the ground but has a fine eye for detail. The fused bullets (Turkish and Anzac bullets that collided and melded into each other, for example, that reveal the stunning intensity of the battles. Naturally, you come away with admiration - not just for some of the extraordinary heroics on both sides but for anyone who came out of the conflict alive. But the overwhelming emotion is anger at the awful futility of it all. Anger that the cream of an infant army should have been so criminally wasted in one of the great stuff-ups in military history.
— The Age Saturday May 13 2006

Other reviews in brief:

Especially evocative.
— Canberra Times
...neatly distils to its essence.
— Adelaide Advertiser
Immensely accessible ... Excellent overall account.
— Sunday Tasmanian