# Reference request: Introduction to Finsler manifolds from the metric geometry point of view (possibly from the Busemann's approach)

This question is a cross post from Math.SE. I have requested the migration of the question, but unfortunately it is not possible after two months of posting. I also have found this related question, but in my opinion it is not a duplicate from mine.

I was reading about geometry in metric spaces from different books, two of them are: (1) A course in metric geometry by Y. Burago, D. Burago and S. Ivanov; and (2) Metric spaces of non-positive curvature by M. Bridson and A. Häfliger. Both develop the Alexandrov's approach to curvature, which uses comparison triangles with the constant curvature model spaces.

For a normed space $$X$$, the following statements are equivalent:

1. $$X$$ has curvature $$\leq\kappa$$ in Alexandrov's sense, for some real number $$\kappa$$.
2. $$X$$ has curvature $$\leq 0$$ in Alexandrov's sense.
3. The norm on $$X$$ is induced by an inner product.

So it seems to me that Alexandrov's approach is not very informative in the normed case. On the other hand, a geodesic space has non-positive curvature in the Busemann's sense if its metric is convex, in general this is a weaker notion than Alexandrov's, and in the normed case the following statements are equivalent:

1. $$X$$ has non-positive curvature in the Busemann's sense.
2. $$X$$ is uniquely geodesic, that is, every pair of points is joined by a unique geodesic (the linear segment between them).
3. $$X$$ is strinctly convex, that is, the ball in $$X$$ is strictly convex which means that for every pair of different vectors $$v$$ and $$w$$ of norm equal to $$1$$ we have that $$tv+(1-t)w$$ has norm strictly less than $$1$$ for every $$t$$ in $$(0,1)$$.

So it seems to me that this weaker notion is the appropriate notion for non-positive curvature in the normed case and I think also for finsler manifolds. I have never studied finsler geometry, but I am very interested in studying metric geometry from this approach. And I do not know where I should start.

My question is: What is a good introductory book about finsler manifolds from the metric geometry point of view? What is a good introductory book for the Busemann's approach? If there was not an introductory book available, a reference to an advanced one along with references that cover the necessary background would be very welcome.

In Math.SE, user @HK Lee has suggested the paper On intrinsic geometry of surfaces in normed spaces by D. Burago and S. Ivanov. And I have found the following references, although I need the advice of the experts:

1. An introductory textbook by A. Papadopoulos about the Busemann's approach: Metric Spaces, convexity and non-positive curvature.

2. A textbook by H. Busemann: The geometry of geodesics

3. Two interesting papers by H. Busemann: The geometry of finsler spaces and Spaces with non-positive curvature.