The description of the book does seem to answer phildwyer's question in full:
But it would be a foolish person who judged a book by the publisher's blurb.
Obviously this book argues one side of the case. Equally obviously, it's very existence proves there is another case to be made:
"Plaid Genedlaethol Cymru's appeal may have been further complicated by the apparent "fascist-style corporatism shown by [Lewis] and other Roman Catholic leaders of the party", according to historian
Lord Morgan.
[27] Author
G. A. Williams characterised the party of the 1930s as a "right wing force", and "Its journal refused to resist
Hitler or
Mussolini, ignored or tolerated anti-Semitism and, in effect, came out in support of
Franco".
[27][28][29][30]
http://en.wikipedia.org/wiki/History_of_Plaid_Cymru
In 1923 Ambrose Bebb wrote in the Plaid journal,
Y Ddraig Goch: "It is a Mussolini that Wales needs."
In 1935 he wrote, in the same journal: "[Mussolini's] was a popular revolution, a people's revolution, and the welfare of every class is the welfare constantly before his eyes.... He influences the nationalist party in Wales."
J.E. Daniel, President of Plaid 1939-43 wrote: "Whatever is the enmity between Fascism and Democracy, it becomes friendship in the face of the great enemy, Communism."
In 1936, Saunders Lewis dismissed reports of Hilter's atrocities as "propaganda."
And here even Wyn Jones concedes Lewis's anti-semitism, and his calls for a negotiated peace with Hitler in 1941-42.
http://www.walesonline.co.uk/news/wales-news/plaid-cymru-fascist-links-claims-5676038
So while this certainly looks like an interesting read, it is very far from being the last word on the question.