The sermons and instructions of Joseph Smith for 1840 are taken from various sources including the Times and Seasons (Nauvoo periodical), the diaries of John Smith, Wilford Woodruff and Joseph Smith, reminiscent accounts and official minutes.
Several new themes surface and some are continued from 1839. Those continued from 1839 include Book of Abraham doctrines (e.g., February 5, 1840 (1)) while new items in 1840 include the emphasis on the resurrection and "eternal judgement" as part of the first principles of the Gospel. Eternal judgement (Heb. 6:1) seems to refer in particular to doctrines regarding sins that are not forgiven, but possibly refers in general to the judgement after death (Rev. 20).
Perhaps the most important new statements are those on the introduction of baptism for the dead (August 15, 1840) and the building of the Nauvoo temple.
One sermon is taken from a written copy prepared in advance (October 5, 1840) on the subject of Priesthood. This is a unique document since Joseph Smith seems to have delivered his speeches extemporaneously as a rule. This sermon clarifies a number of issues related to priesthood and priesthood offices. Together with D&C 107:1-58 it became the standard for doctrine on the subject.
All the sources for this year appear in WJS