Is "Functional Analysis" by "Yosida" a good book for self study? I was wishing to start studying by myself the book Functional Analysis by Yosida, does anyone have already used it, is it a good reference?

Yosida's book is excellent, but certainly not easy reading. It was absolutely cutting edge when it was first published and the style is demanding. I would only recommend it as second or third reading in functional analysis after having achieved a pretty solid background. In addition, some people complain about difficulties understanding the sometimes slightly outdated terminology.
On the positive side, Yosida contains many results and examples that are otherwise hard to find. This is one of the reasons that it still is a widely used reference book which has stood its test of time. Familiarity with it can't hurt...
Since your previous questions indicate that you are relatively new to functional analysis, I would recommend to read something easier going and maybe more modern. You might enjoy Lax's book or Stein and Shakarchi or Reed and Simon. See also


*

*Good book for self study of functional analysis

*An introductory textbook on functional analysis and operator theory
for further recommendations.
