A large part of my master's thesis (~25%) is essentially a summary of one book. How to cite correctly and in a tastefull manner?

Overriding answer: ask your advisor.

In expository work like this, what I've usually seen done is that you start the section with a note saying something like "The material in this section is primarily drawn from Handbook of Reticulated Splines by P. Smith [47]." After that, you don't bother to decorate every sentence with "[47]" or "as Smith states", except where you feel it is particularly important for clarity or the reader's convenience (e.g. you want to quote Smith's exact words, or point the reader to something on a specific page of the book, or you want to discuss Smith's approach in contrast to someone else's).

But again, ask your advisor in case they have some other ideas as to how you should handle it.

Are you sure you can retell what's in that book in a better way than the author wrote it in the first place? If so, proceed as you described. If not, I suggest you pull out very little from that book to put into your thesis, but encourage the reader to read the reference.

Your job, in writing your thesis, is to present your original contribution. You must present some background for that, of course, but not to the point of rewriting another work, unless there's something inscrutable about the other work, and you need to rewrite it to make it understandable.

Now, if you wanted to critique some ideas presented in that book, or go into some aspect(s) in greater depth -- that might be a different matter.