I used to go out and eat alone sometimes, go out and eat with colleagues some times, stay in and eat alone sometimes, stay in and eat with colleagues sometimes (that last would usually be round a table in the office, not in an canteen or works cafeteria). Relatively frequencies depended on circumstances (like where I was working, how many people there were in teh workplace, and so on).
I never did any technical reading a lunchtime. I don't mind working long hours when it's needed, I don't mind working or reading technical stuff after dinner or before breakfast, I don't even mind skipping lunch to get on with work on the rare occassions when it's necessary; but if I take a lunch break I want to forget things technical and relax; when I was lunching alone, I would read either fiction or a non-technical newspaper, or take a walk, or go lie on the grass in the sun, or go into a pub and play pub games (things like shoveha'penny, pitch penny, bar billiards, devil among the taylors and so on) with the locals. When I was lunching with colleagues, we would play games (traditional pub games or things like crib, dominoes, or word games or whatever) and/or talk: conversation would usually be gossip about friends/acquantances/colleagues/PHBs and other burdens/news/films/music - anything but hardcore technical shop (except at one place where we used to take multi-hour lunches; much of that lunch time was spent roughing out system structure diagrams, bits of maths, and odd code fragments on beermats and table napkins, but that still left time for non-technical relaxation during that long lunch break).