One of the best books I have read about Abraham Lincoln was Lincoln's Melancholy: How Depression Challenged a President and Fueled His Greatness by Joshua Wolf Shenk. It shows how Lincoln used humor, empathy, and endurance to manage his chronic mood disorder. All traits that helped him lead the Union through the Civil War. https://www.amazon.com/Lincolns-Melancholy-Depression-Challenged-President/dp/0618773444
Yeah I got first wind of this when reading Lincoln's Melancholy and there are some very personal (but not sexual) letters between Lincoln and Speed. To be fair it was very common for lawyers on the same circuit to share accommodation, so that shouldn't be taken as evidence, but some people point to Lincoln's extremely close friendship to Speed (his depression would recur when Speed was away, etc) and relative "detachment" from women, and think there could be more there. Be wary if you go reading into it because there's some letters/diaries out there that are definite hoaxes.
edit to add: Here is an example of one of the letters, unfortunately I can't find the one I'm thinking of where Lincoln expresses depression from missing Speed. You'll see that it indicates a very close relationship (where Lincoln straight up asks Speed if he regrets getting married) but not necessarily romantic. That said, it's clear they confided intimately in each other.
Abraham Lincoln was severely depressed.
https://www.amazon.com/Lincolns-Melancholy-Depression-Challenged-President/dp/0618773444