If you want to focus on one PERIOD of his life, I would suggest the period just PRECEDING his Presidency -- how he became a champion of the young anti-slavery Republican Party in the 1850s, beginning with his reaction to the Kansas-Nebraska Act in 1854, and featuring the Lincoln-Douglas debates of 1858 (including the interesting EARLIER Senate race Lincoln nearly won)
A couple of very solid sources for that:
** William Lee Miller, *Lincoln's Virtues: An Ethical Biography* (2002) - the second half of the book is focused on this period (and the earlier stuff is also helpful)
** John Waugh, *One Man Great Enough* (2007) - his life up to June 1861 (death of Stephen Douglas), a very good read
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You might also choose to do something on his Presidency, but instead of doing the whole thing, focus on some specific aspect of what he did. Two worth considering:
(1) Lincoln & freedom -- not just the Emancipation Proclamation, but how that fit into a SERIES of efforts by Lincoln to deal with the slave issue, starting with his attempts to sell the border states on accepting a plan of 'compensated emancipation' (they refused) and ending with his campaign for the 13th amendment
Best sources:
** THE definitive book on the Emancipation Proclamation and related material is Allen Guelzo's *Lincoln's Emancipation Proclamation: The End of Slavery in America.* (2004).
** a wonderful web site -
http://www.mrlincolnandfreedom.org/inside.asp?ID=27&subjectID=3
check out this related site for lots of other Lincoln stuff:
http://www.abrahamlincolnsclassroom.org/Library/newsletter.asp?ID=1&CRLI=64
(2) Lincoln's powerful use of WORDS
- lots of recent stuff on this. A great place to start is book by Ronald C. White Jr. *The Eloquent President* (2005), which explains his major speeches from his election as President to the end. White also has an excellent book on Lincoln's Second Inaugural (and LOTS of folks have written on the Gettysburg Address - which the first book above will point you to).
A couple of the most important EARLIER speeches:
"House Divided" speech (June 1858, launched his Senate campaign vs. Stephen Douglas)
"Cooper Union Address" (Feb 1860) - very thoroughly researched argument that the founders meant for slavery to be limited and to die out; impressed people in the Northeast, which eventually won him the support for the Presidential nomination (he was everyone's 'second choice')