If your interest is in Alan Turing, and you are only just becoming familiar with him, you would probably be better served reading what many regard as not only an excellent biography of Alan Turing, but an excellent piece of biographical writing in and of itself, Andrews Hodges' ALAN TURING: THE ENIGMA. The current copy on the American Amazon site is an expanded and pricey edition, so you may want to go to the the United Kingdom Amazon site to get the slimmer, earlier edition, which not only costs less, but was also the basis for the award-winning play BREAKING THE CODE. After you've read Hodges' work, and wish to read more about Turing and his work and theories from other perspectives, then you may want to avail yourself on some of these other texts.