I used to read one book at a time, and only one book.
I liked the system and it worked for me for ages. Then it sort of changed. I still read to my kids every night and we all love it, so that's two books there, then I usually have one in Catalan underway, and a Spanish book - theoretically being read (though it is at the very bottom of the pile) but sometimes I just don't feel like any of the books I'm reading. So I pick a book based on mood.
I think this is also because I am sometimes not reading any book that I LOVE.
Right now I'm reading The Book with No Name as recommended by Eldest, though I am finding it a wee bit to complicated for a light read and way too gory. Whether I finish is yet to be seen and the sheer violence is kinda slowing me down. I am also reading Everything is Illuminated by Jonathan Safran Foer, and it is also annoying me, as half the book is written in the kind of bad English I have to mark all year, and it is spoiling the joy, to say the least.
I am reading an Isabelle Allende, whose books are fine, but I really want to read it in Spanish not English so I am hesitating. I am reading El cel és blau, la terra blanca by Hiromi Kawakami in Catalan, and that is good, but not cheery, and finally Kon Tiki in Spanish (remember that bottom of the pile?)
I think it is a sign of not having a book going that is really grabbing me, that I simply cannot put down. Though the Catalan maybe would be doing so if it were slightly easier to read Catalan...though I am getting pretty good at it now anyway.
So, do you read one at a time? As the mood takes you?
Does it vary?