probably books
Best way to improve?

3 things:
#1 - Study good books
#2 - Analyze your own games (with help from others on chess.com forums? )
#3 - Analyze master+ games (billwall and jimwest and others on this site do a great job with this!)

Tactics, tactics, tactics! This is the best and most efficient way to improve. I can say by doing chess tactical puzzles each day has really helped me and it has shown through my tournament results. Unless you are +2100 level, 9 times out of 10 the game will come down to whoever sees the tactics better.
The tool I use to improve my chess tactics is the "Chess tactics server", you should search this on google or any other search engine and you will find it. I usually do about 100 of those puzzles each day. It really is great stuff!


I have a differing opinion to that of IronRaven. Nothing personal, but I believe tactics is the way to go, as MattHelfst has pointed out. Tactics actually combinative vision, as well as your memory. And it is quite fun too, if you really have a love of chess. What is better than solving a difficult puzzle whereby all the moves are just miraculous?
Tactics can be said to be the building blocks of chess. Without them, you are almost condemned to failure, as you simply cant outplay someone who can see a forced mate in 6 or a combination that wins a piece in 7 moves! Of course, the positional element and technical element of Chess cannot be said to be unimportant, but in my opinion those can wait until you have reached a higher level and are able to appreciate the more intangible aspects such as positional advantage.
As for ways to train, as Erik has mentioned, Books are definitely useful. Annotating and analysing your own games are another way, and perhaps some master games. However, it is advisable not to go too deep into very complicated master games as you would probably not be able to have any idea what is going on! Another way is to have a chess engine to help you in your analysis.
One book I can recommend strongly is Nimzowitsch's "My system". It is a very good and substantiated books which can be used from the beginner to even master level! The difficulty level progresses up the book and the basic fundamentals and ideas are clearly explained in this book.
What is the best tool to improve ones chess ability?