The Sources of Islam
is Tisdall's earlier book (from 1900) on the same topic. It
contains already most of the important material, but being only a third
in size (about 100 pages) is not as thoroughly documented as the later
edition (it also has a few minor mistakes that have been corrected in
the later edition), but this is the version that has been reprinted
many times, probably because it is much quicker to read.