Besides Onepence's excellent (and officially recommended) suggestion,
Gleanings from the Writings of Baha'u'llah covers in excerpt form. The introduction to the book describes sections
70 to 99 as being about the soul (if that's what you want to focus on) (though this categorization may only be approximate). But the style of English used in the translations of Baha'u'llah may need a little getting used to, though I think you will enjoy it thoroughly once you have. However, a light but meaningful introduction such as in "Baha'u'llah and the New Era" is a great beginning, especially since you may especially be interested in getting a good overview first before going too in depth.
Also, if you are open to meeting other Baha'is, we should recommend going through a training institute course in your locality. You can study a sequence of Baha'i courses with others (and in turn be better prepared to help others if you come to accept the Faith, as you are welcome at any time to do).
all the best,
Brett
