I'm a beginner, and sometimes I lost my games because my territories are smaller respect to my opponents. Maybe because in the first moves I cannot build a good territories in an angle, and sometimes because I have difficult to make an invasion in my opponent territory. Where I can find good book that can help me to improve my strategy? I live in Europe, so I need book easily available here. Thanks!
Hi, if you are a beginner you need to learn joseki and some techniques first. You can try some websites like eidogo.com (this is very useful!).
If you prefer books:
Dictionary of basic joseki vol.1-3 - Ishida Yoshio
Basic Techniques of Go - Haruyama Isamu & Nagahara Yoshiaki (this one is maybe too difficult for you right now, in few months it will be great!)
If you're beginner, joseki dictionary is definitely not a way to go. It'll be overwhelming, and most of your opponents won't follow joseki anyway, and you'll have no idea how to proceed after that.
I second "Basic Techniques of Go" recommendation. Also Lessons In The Fundamentals Of Go (http://senseis.xmp.net/?LessonsInTheFundamentalsOfGo) is considered by many as a must-read. One more book I'd like to recommend is "Opening Theory Made Easy" by Otake Hideo.
All of those books should be available in Europe from Go Shop Keima, for example (google for it). (By the way, you can check there "Go books by level"; the site considers Ishida's joseki dictionary to be for 5k and stronger.)
Perhaps you could learn even more if you add the reading of The Second Book Of Go by Richard Bozulich. It explains in very good detail what you need to improve in your games.
I second Bozulich's The Second Book Of Go. It helped me improve a lot, and it covers most of the aspects of your game that you'll need to improve as a beginner
Related to the main topic, but I am also a beginner, and I was wondering if anyone had some tips on the following situation. My opponents are sometimes very aggressively going for influence in the middle. I counter by getting the corners, but I have trouble invading once they have a very large circle in the middle. Is there a golden rule like "If your opponent goes for influence, always keep some stones in or near the middle for late game invasions" ?
Thanks in advance for the help!