For a popular history, Mark Kurlansky's The Basque History of the World is very accessible--though I don't have access to see how it has been received by academic reviewers. Personally, I found little objectionable in the work, and its pro-Basque bias is mild enough to be easily borne. It is also highly available.

More difficult to come by is Escape Via Berlin: Eluding Franco in Hitler's Europe by José Antonio Aguirre y Lecube, which largely serves as his autobiography. As he was the president of the Basques starting in 1936, this book covers his brief presidency in Spain as well as his exile from there. The title refers to the incredible true story of his days as a refugee in France, nearly making it to Dunkirk but coming up just short, and then actually traveling to Berlin of all places to escape Hitler and Franco by traveling to Sweden. The book is fairly blatant propaganda, but it also contains a good deal of facts. To his credit, Aguirre is candid about some of the failings of his presidency as well as his ignorance of a Vatican peace proposal during the Spanish Civil War.

There are also portions on the Basques in the three main books on the Spanish Civil War: Beevor's Battle for Spain, Thomas' The Spanish Civil War, and Preston's The Spanish Civil War. All three are excellent, but I would advise just borrowing them from the library as all three only have small sections on the Basques. Buy them later if they catch your interest.