Three books were written by the series script editor and principle script-writer - the late great John Brason - to accompany each series of Secret Army:

SECRET ARMY
by John Brason, BBC Books, 1977, out of print.
A prequel to the first series which introduces all the major characters.

SECRET ARMY DOSSIER
by John Brason, BBC Books, 1978, out of print.
Adaptations of some of his first and second series scripts (includes: Good Friday, Be the First Kid in Your Block to Rule the World, Russian Roulette and Day of Wrath) and some new material (Pastures New and Phoenix).

SECRET ARMY: THE END OF THE LINE
by John Brason, Star Books, 1981, out of print.
Adaptations of his third series scripts.

All of these books can be regularly found for sale on ebay.

Also worth a read is the autobiography of the series' Technical Advisor - Group Captain Bill Randle and his first novel: Kondor.

BLUE SKIES AND DARK NIGHTS
by Bill Randle, Independent Books, 2002.
Includes a very readable account of his evasion experience which inspired many of the series' characters and situations. Click here to buy this book on Amazon.

KONDOR
by Bill Randle, Independent Books, 1999.
Fiction. The story of 'Operation Kondor', a German attempt to infiltrate a resistance evasion line. Click here to buy this book on Amazon.