Dehn's work is centred around the deep and beautiful harmony between topology and combinatorial group theory. The connection manifests itself in the study of curves on surfaces. Here two central group theoretical problems crystallises: the word problem (corresponding to the topological problem of deciding whether a curve contracts to a point) and the conjugacy problem (topologically, to decide whether two given curves are conjugate). Dehn illuminates the situation with the wonderful idea of the group diagram. The investigations are given an even more geometrical feel through the use of hyperbolic geometry, which is prompted from within both group theory (the natural domain in which to draw many group diagrams) and topology (universal covering surface). We get to meet all these things in the form of lectures delivered before the publications. But these ideas have much greater scope and power, as Dehn shows in his impressive paper "On the Topology of three-dimensional Space" (1910). Here, of course, knots become important. This will soon (1912) lead Dehn to add a third item to his list of the fundamental problems of combinatorial group theory: the isomorphism problem. But wait a minute, you say, even a solution to this problem would not suffice to tell all knots apart. Well, Dehn is way ahead of you: in his 1914 paper he shows that the trefoil knot cannot be deformed into its mirror image. The two last papers are on mapping class groups of surfaces. Perhaps Dehn wanted to repeat his early success --the fountain of youth---to again come up with a clever combinatorialisation, this time of the mapping class group. I for one was less impressed the second time around.

There are introductions to each paper experts who just want to read some beautiful mathematics, Stillwell's book "Classical Topology and Combinatorial Group Theory" is an excellent guide.