When Hugh Thomas first published his monumental history The Spanish Civil War, he was just 30 years old. It was the first truly comprehensive study of what later writers have called "the last great cause," and it was stunning for its scope and for the sober approach to a subject that had produced so much passion. As was only natural for such a vast work written so near in time to the events it depicted, it contained numerous errors of fact and judgment.

Now, 16 years later, Thomas has substantially revised and enlarged his history. Taking advantage of a considerable amount of new original research, and the availability of dozens of books by others that have been published since Thomas wrote his, this new edition is probably the most valuable work in English on the war.

Perhaps the most important change from the earlier edition to this one is Thomas's recognition that the Franco military dictatorship was typical of modern Spanish history and that the republic of 1931 to 1936 was the abberation. There are other philosophical changes, as well, and changes in the assessment of key figures such as republican President Manuel Azana.

The book has been rewritten thoroughly and everywhere details have been changed to conform with newly available information. The old version, for example, had the war front twice as far from the Basque town of Guernica - which the German Condor Legion destroyed in the world's first mass air attack - than the new edition. And the "postmaster" of a Spanish resort village called Altea, who was supposedly axed to death by an anarchist, has become a "cafe proprietor." The latter may seem trivial, but for someone who has tried to check out the Altea story firsthand and failed, it provides a clue of considerable interest. (Harper & Row, $27.50)