I see that most of the usual suspects (Beevor, Thomas, Preston etc) have been mentioned already, though there's plenty of other decent overaching accounts from a variety of perspectives (from Stanley Payne, Raymond Carr, Burnett Bolloten, Mary Vincent, Frances Lannon, Gabriel Jackson and others). There are also any number of first hand accounts (beyond Orwell and Hemingway), from the likes of Franz Borkenau, Gerald Brenan, Arthur Koestler and many others.

In terms of slightly more specialist stuff which is still engaging (heavily biased towards my own interests, which reflect the international dimensions of the conflict), I'd consider:

• Graham, Helen, The War and its Shadow: Spain’s Civil War in Europe’s Long Twentieth Century (Brighton, 2012).
• Keene, Judith, Fghting For Franco: International Volunteers in nationalist Spain, 1936–39 (London, 2001).
• Kirschenbaum, Lisa, International Communism and the Spanish Civil War (Cambridge, 2015).

I found Antony Beevor's The Battle for Spain to be a gripping read and a good introduction to the period. Beevor is a military historian, so he focuses more on the battles than say the social or cultural history, so depending on what your mother likes it might be a bit too nitty-gritty on the details of combat/armies, but Beevor is fairly well-rounded and an impartial historian.