Question: Hello,

I'd like to hear your opinion for ergodic theory books which would suit a beginner (with background in measure theory, real analysis and topological groups). I am looking for something well structured, well motivated, and perhaps with application to other fields.

any such book exists?

I tried a book by nadkarni, and could not read through it, seemed to concise to me, and tried the book by Petersen which I felt was accessible but didn't follow a clear path, jumping from subject to subject with lots of different object or properties.

What are your recommendations on the subject?

Answer: 
What about the following?

Sinai, Ya. G.
Introduction to ergodic theory.
Translated by V. Scheffer. Mathematical Notes, 18. Princeton University Press, Princeton, N.J., 1976. 144 pp. ISBN: 0-691-08182-4

This seems to have the highest content-to-volume ratio. It treats, among others, invariant measures, translations on compact abelian groups, geodesic flows on Riemannian manifolds; it gives applications to number theory and discusses ergodic theory of ideal gas (as applications to "other fields" that you may be interested in). Two chapters deal with entropy.  

The proofs are not always carried out in full detail, though.
I have learned topological dynamics from this [textbook](http://www.cts.cuni.cz/~kurka/studij.html).