I have a real newbie question here.
Is it true that if we have a diffeomorphism of a complex manifold that preserves induced almost complex structure than it is a biholomorphism? (I assume that the inverse map also preserves almost complex structure).
It also would be very useful if somebody could give some basic references so I can look up similar simple questions there.
Yes, it is true, it is a corollary of the inverse function theorem in complex analysis. If it is a diffeomorphism, locally, its complex Jacobian is invertible and you can apply:
https://en.wikipedia.org/wiki/Inverse_function_theorem#Holomorphic_functions