Road map to learn about $\mathrm{Out}{F_n}$

Here are some assorted recommendations.

• Stallings's "Topology of Finite Graphs" and Bestvina's course notes "Folding Graphs and Applications" are a great introduction to a technique that has found wide-ranging applications, both in the study of $\operatorname{Out}(F_n)$ and more broadly.
• Vogtmann's "What Is Outer Space" makes a great companion to the Bestvina survey.
• Culler and Vogtmann's "Moduli of Graphs and Automorphisms of Free Groups" is maybe the beginning of the modern perspective on $\operatorname{Out}(F_n)$ and also a really beautiful paper.
• Bestvina and Handel's "Train Tracks and Automorphisms of Free Groups," with the Culler–Vogtmann paper, are perhaps the biggest source of tools for the study of $\operatorname{Out}(F_n)$. The later series on "The Tits Alternative for $\operatorname{Out}(F_n)$" with Feighn and the more recent Feighn–Handel "The Recognition Theorem for $\operatorname{Out}(F_n)$" build on this technology.

The above are sort of what I would consider the backbone of the theory. From there are the various hyperbolic complexes associated to $\operatorname{Out}(F_n)$, there's the study of geodesic currents or the bordification of Outer Space, and numerous other papers and related topics that grew out of this study.

$\operatorname{Out}(F_n)$ is a very interesting group in part because many people who study it do so by ruthlessly analogizing with the mapping class group of a surface or with linear groups in order to get a feeling for what ought or ought not to be the case. I think in order to understand the perspective of people working in the field, it helps to be open to broadening your understanding of other areas.

Actually, that last sentence applies to geometric group theory writ large: the field grew out of tools and insights developed to capture the essence of behaviour that people noticed in a wide variety of subfields of group theory, geometry and topology. Yours and my generation is peculiar in part because we began by studying geometric group theory as its own discipline. I'm not sure anything is lost here per se, but it can help to understand the perspective of the people whose work ours builds off of.

I'm obliged to add "The topology, geometry, and dynamics of free groups. Part I: Outer space, fold paths, and the Nielsen/Whitehead problems" by Lee Mosher which I find very helpful (looking forward to Parts II & III).