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:

- $X$ has curvature $\leq\kappa$ in Alexandrov's sense, for some real number $\kappa$.
- $X$ has curvature $\leq 0$ in Alexandrov's sense.
- 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:

- $X$ has non-positive curvature in the Busemann's sense.
- $X$ is uniquely geodesic, that is, every pair of points is joined by a unique geodesic (the linear segment between them).
- $X$ 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:

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

A textbook by H. Busemann: The geometry of geodesics

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

Thanks in advance!