  A map is a partition of the sphere into regions that are labeled as countries
or holes. The vertices of a map graph are the countries of a map. There is an
edge if and only if the countries are adjacent and meet in at least one point.
For a k-map graph, at most k countries meet in a point. A graph is k-planar if
it can be drawn in the plane with at most k crossings per edge. A p-page book
embedding of a graph is a linear ordering of the vertices and an assignment of
the edges to p pages, so that there is no conflict for edges assigned to the
same page. The minimum number of pages is the book thickness of a graph, also
known as stack number or page number. We show that every k-map graph has a book
embedding in $6\lfloor k/2 \rfloor+5$ pages, which, for n-vertex graphs, can be
computed in O(kn) time from its map. Our result improves the best known upper
bound. Towards a lower bound, it is shown that some k-map graphs need $\lfloor
3k/4 \rfloor$ pages. In passing, we obtain an improved upper bound of eleven
pages for 1-planar graphs, which are subgraphs of 4-map graphs, and of 17 pages
for optimal 2-planar graphs.
