Here is a list of all files with brief descriptions:

break.h
command/break.h
command/help/break.h
buildargv.h
chdir.h
help/chdir.h
cmd.h
commands.h
comment.h
help/comment.h
config.h
continue.h
help/continue.h
debug.h
debugger.h
delete.h
help/delete.h
dep.h
down.h
help/down.h
edit.h
help/edit.h
eval.h
debugger/command/expand.h
debugger/command/help/expand.h
expand.h
file.h	Definition of target file data structures for GNU Make
file2line.h
file_basic.h
filedef.h
finish.h
help/finish.h
fns.h
frame.h
help/frame.h
function.h	Builtin function expansion for GNU Make
getopt.h
gettext.h
globals.h
gmk-default.h
gnuremake.h
hash.h
help/help.h
help.h
implicit.h	Header for impilict.c
command/help/info.h
command/info.h
info.h
job.h
help/list.h
list.h
help/load.h
load.h
main.h
make.h
makeint.h
mock.h
msg.h
help/next.h
next.h
os.h
output.h
debugger/command/help/print.h
debugger/command/print.h
print.h
profile.h
help/pwd.h
pwd.h
help/quit.h
quit.h
read.h
rule.h
help/run.h
run.h
help/set.h
set.h
help/setq.h
setq.h
help/setqx.h
setqx.h
help/shell.h
shell.h
help/show.h
show.h
help/skip.h
skip.h
help/source.h
source.h
stack.h
help/step.h
step.h
subcmd.h
help/target.h
target.h
trace.h	Header for routines related to tracing and debugging support
types.h	Miscellaneous types
help/up.h
up.h
variable.h
vpath.h
help/where.h
where.h
help/write.h
write.h