I prefer software, especially the chess tactics for beginners, intermediante and advanced series, for a number of reasons
1) There is no need to go thumbing all the way in the back of a book to find the answer
2) I can play out my own variations and see why they don't work- sometimes when I'm reading a tactics book, I think "But what if I do that? Then what will he do?"
3) It gives partial hints so it's not an all or nothing situation
4) The hints actually help increase tactical ability i.e., it shows you the lines of attack
5) I can see my relative rating to see my progress
6) It times how long it takes me to solve them, which is useful.
What's your opinion? Books or software?