There are many Bahai books online, free
For Bahai scriptures and some other bits and pieces, see the Bahai reference library:
http://reference.bahai.org/en/ Among Baha'u'llah's works (see
http://reference.bahai.org/en/t/b/), look at the Hidden Words and Seven Valleys, Gleanings, Tablets revealed after the Aqdas
Among Abdu'l-Baha's works, (
http://reference.bahai.org/en/t/ab/), I recommend
Selections from the Writings of Abdu'l-Baha, Some Answered Questions
For Shoghi Effendi, I suggest The Promised Day is Come.
All these and much much more in secondary literature is online at the Bahai Library Online,
http://bahai-library.com/For brief introductions, there is Momen's book:
http://bahai-library.com/momen_short_introduction_bahaiand a 'brief course' compiled by the Bahai World Centre:
http://bahai-library.com/brief_course_bahai_faithIf you really like a solid book in your hands, there is Garlington's "Bahai Faith in America", Momen's title that I forgot, McMullen's "the Bahai" (sociological), and Peter Smith's "the Babi and Bahai religions" (a sociologically-tinted academic introduction & history). If I was teaching Comp. Rel. 101 I would chose Peter Smith's book