Blowing the dust off this old thread, as I've recently bought an Alfa book not mentioned here. I already had the Borgeson "Alfa Romeo Tradition," which is pretty good. It follows the history by naming each era after one of the major personalities.
I've recently bought a second hand copy of "First Among Champions, The Alfa Romeo Grand Prix Cars" by David Venables. He's an excellent historian, and goes deep into the early days of the firm. Bought it because his other books are great. It's second hand and, believe it or not, cheap at £48. There are copies going for three times that, so I was prepared for it to be well worn, but in fact it's immaculate.
Realise not everyone is heavy into racing, and it's a deep read - lots of facts, detail, text without headings and so one - but I'm loving it. Did you know that when Merosi wasn't paid... oh, never mind, you'll have to read it.