That means shockingly little of what is written about him is grounded in a measured reading of his actual life, not even the stuff by professional historians. In fact professional historians are often among the worst offenders.
If you want a measured biography of Lincoln try William Herndon's "Life of Lincoln," Ward Hill Lamon's "Recollections of Lincoln," or Gideon Welles' collected essays on the administration of Abraham Lincoln. All three were written by people who knew Lincoln personally, and they were each written with the intent of humanizing their old friend in the wake of the post-mortem apotheosis of his legacy that happened when the Radical Republicans of reconstruction seized upon his name as a political tool for their policies.
None of these biographies is perfect, but they are closer to the "real" Abraham Lincoln than most of what's written about him today. Herndon was Lincoln's law partner back in Illinois and did a lot of primary source research on Lincoln's pre-presidential life.
Lamon was an old Illinois companion of Lincoln's who accompanied him to Washington and was something of Lincoln's go-to guy for political dirty work during his presidency. As such, he was a bit of a scoundrel in his own right but he was among Lincoln's closest confidants in life. He's not very popular with the Lincoln hagiographers because he presents his subject in a less than saintly, though generally positive, light (Lamon's Lincoln is the type who liked bawdy jokes and wasn't afraid to get his hands dirty in political scheming - hardly the stuff of marble column heroes). But he is, IMO, more accurate than most.
Welles was Lincoln's secretary of the navy and a "moderate" on the cabinet who resisted the radical attempts to claim Lincoln as one of their own after his death. Unfortunately his essays have been out of print for many decades and may be hard to get a hold of.
You might also try Nicolay and Hay's multi-volume biography of Lincoln, though I'd attach a word of caution about it too. Nicolay and Hay were two of Lincoln's personal secretaries and bore eyewitness to much of his presidency. But they were both very young during their employ, and they tended to look on their boss through biased and almost worshipful lenses - much as an intern fresh out of college looks up to the "powerful" congressman who employs him. Whereas Lamon knew Lincoln as a friend and peer before he reached office, Hay and Nicolay knew him only as a superior and were devoutly loyal to upholding his reputation - even to the point that they got into a public spat with Lamon about it in the 1880's as to who was representing the "true" Lincoln.
But that said, steer clear of both the glowing Lincoln-worship bios and the revisionary Lincoln-bashing responses they provoked. The both present Lincoln as a hero or a villain, but lost in the process is Lincoln as a human being. That Lincoln is neither wholly good nor evil, a masterful and skilled politician-orator, and a man with both qualities and faults. His faults do extent to some aspects of the way he governed, and those tend to either be glossed over or harped upon by most writers. I'd therefore advise your friends not, as some here are suggesting, to excuse away those faults, but to concede your own cognizance of them and use that as a vehicle to reorient their perspective from the flawed "either worship Lincoln or hate Lincoln" dichotomy of choices.
John Nicolay also wrote The Outbreak of Rebellion, which seethes with his personal animus and partisanship but also allows a very frank look at some things that were done in the Government. His animation sometimes carries him away, and at such moments some candid comments may pop out.
His book is good for following the outbreak of hostilities, and it is helpful to anyone trying to penetrate the shadows and murk with which Lincoln sought to conceal his intentions and his hand during the opening moves of the commencement of the war.