What is the best method improve chess calculation skills？

Play 30 min so you have time to calculate and you will start doing it better and faster

Could you suggest a beginners' or relatively easy book on endgame studies? Thank you in advance!

You might want to look at Chess Endgame Quiz by Larry Evans. It contains 200 positions grouped by K+P endings, R+P endings, Q endings, and minor piece endings. Most positions are from studies, but some are from actual games. Some are easy (the first position involves basic opposition, for example), many are intermediate, and some are quite difficult. The book costs $12.78 on Amazon, less for a used copy.

With this or any endgame book, you should set up the position on a real board and work out the solution before looking at any answers or explanations. Write down your analysis and then compare it to what the author says. You will learn a lot this way.

I'm not sure that I understand. Here's an example: position 15 in Evans's book says "Composed by Grigoriev" and gives this position, with White to play and win:

This is one of many positions in the book with the notation "Composed by ..." That's what I was calling a study. Is composition a more accurate word?

Either way, I am finding these positions to be very helpful in practicing calculation and in improving my understanding of endgames.

Ka6?

Yes, Ka6 is correct.

домофон

The most important thing is knowing when a position requires concrete calculation, as opposed to positions that do not -- sometimes all the position requires is that you make a good move; there is no great one to be made.  The problem is that during a game there is no neon sign that pops up and says "White to move and win."

Playing through tactical exercises may help your calculation, but it will help your recongition much more.  It will help you learn to spot potential opportunities -- tell you when to calculate.  For that reason, it is more important to look at many tactical puzzles per hour of study, as opposed to trying to solve every puzzle and end up looking at fewer per hour.

I know of two excellent books that discuss the process of calculation: 1) Power Chess Moves by Charles Hertan and 2) The Inner Game of Chess by Andrew Soltis.  Hertan's approach is more practical in an era of faster time controls.  His book helps you train your tactical vision so that you can quickly determine if an opportunity exists, and, if not, just get on with the game.  Soltis' book is more applicable to the days gone by, when GMs used to play with time controls of 40 moves in 2 1/2 hours.  But he does give a lot of examples if when and why our calculation can go awry ( such as shifting pawn structures in the middle of a combination.)

@DeirdreSkye: Thank you for taking the time for that explanation. I appreciate your willingness to share your knowledge.

I need to get Kasparian's book. I have been doing some work on endgames, but am getting a greater appreciation for their importance. In the past month, I have drawn two OTB endgames that I should have won, and managed to draw another with a pawn down against a 1994 rated player because it was a R+P ending that I had studied and understood how to play. The lesson for me and anyone willing to listen is to spend a significant percentage of study time on endings.