I am writing an essay on Abraham Lincoln where I need to assess his successes and failures. I need to review an academic expert's opinion of Lincoln, quoting an extract that I will then analyze the value of. Any helpful information or direction to quality academic sources on Lincoln would be very helpful.

It is fairly easy to list Abraham Lincoln's successes. He led the United States through its darkest hour, the Civil War, from 1861 to 1865. This was not at all inevitable, as the country was more divided than is sometimes remembered. Many people in the North were opposed to the war, and widespread dissent was not uncommon, most famously with the draft riots that gripped New York City in 1863.
Lincoln won an election in 1864 in the midst of the tension leading up to the Civil War. Some historians even credit his election as one of the reasons for the outbreak of war. He proved an adept strategist, though he sometimes struggled to identify good military leaders to command the armies of the Union, a fact that might be considered among his failures as President. Generally, though, Lincoln's untimely assassination perhaps spared him criticism for major failures. Soon after having won the bloodiest war in American history, he died.
As for scholarship on Lincoln, there are many to choose from. Indeed, perhaps more books have been written about him than any other American (president or otherwise), and many historians specializing in the Civil War era have written one. James Macpherson's Tried By War is an excellent account of Lincoln's abilities as a commander-in-chief, while The Fiery Trial by Eric Foner is an important book about Lincoln's evolving position toward slavery. You might also consult Macpherson's masterpiece Battle Cry of Freedom for analysis of Lincoln's performance during the war.