It highly depends on what you want to achieve, on the jos main site there are references to books that might be of interest to you, also the jos library is filled with good stuff to read.
Personally i like books like ''The book of 5 rings'' and ''Meditations from Marcus Aurelius'' quite interesting and very helpful. Also books like ''Mythos'', ''Tao Te Ching'' and ''the art of war'' are fun reads in my opinion.