This two-book set features John Wilson's Lost Cause, from Seven (the series), and Broken Arrow (The Seven Sequels). In Lost Cause, Steve goes to Spain after his grandfather's death, meets a Spanish girl named Laia and learns about his grandfather's involvement in the Spanish Civil War. Broken Arrow takes him back to Spain (and Laia) for a crash course in espionage and further revelations about Grandpa's role in the Cold War.
Check out The Missing Skull by John Wilson (the Seven Prequels) and all the other books in Seven (the series), The Seven Sequels and The Seven Prequels.
This edition is not currently available in bookstores. Check your local library or search for used copies at Abebooks.