There was another book full of Arrau talking to Joseph Horowitz: Conversations with Arrau. He admits to being John Travolta fan in it, so it's worth getting.
His penchant for ponderous tempos may be because he thought that faster tempos trivalise the music. The slow tempos often work like hell, esp. in Liszt (Benediction de Dieu, Jeux d'eau) and his playing often sounds somehow very deep, e.g. in the Liszt Sonata or Harmonies du Soir.
On the downside, I don't know if I go for his touch/tone much. He's somehow pretty heavy-handed, maybe too much so in some Chopin pieces. Maybe his Chopin also often gets slammed because it's a bit under-nuanced sometimes. That said, I think his Concert Allegro is wonderful. He does some nice rubatos in it.
He did have a big technique (e.g. his Islamey, his Wilde Jagd and his really powerful Chasse Niege), and he made things difficult for himself by not cheating and by using awkward fingerings to get particular effects (e.g. he fingered the argeggio at the end of the development in the Appassionata 3rd movt with 1-2-1-2 etc).
There are thus plenty of reasons for forgiving him for playing Feux Follets at half tempo.
Topic: Any fellow Claudio Arrau fans?