This is a nice article looking at the primary sources around Lincoln's last days with a critical eye:
My new book project, a study of Lincoln’s time at City Point in March/April 1865, presents me with another oft-used famous memoir and often (in my opinion) too good at times. William H. Crook, self-described as Lincoln’s bodyguard, produced some articles in 1906 about his time with Lincoln at City Point that later appeared in a larger book, Through Five Administrations (1910). (All are said to be his recollections as "compiled and written down by Margarita Spalding Gerry.") Excerpts from these works were widely serialized in newspapers in the years following their publication, further cementing their stature in the public mind. Still, the question remains: Was Crook even present during Lincoln’s visit to City Point?
I am not the first historian to question Crook’s account(s). A number have pointed out that other, more reliable first-hand encounters with Lincoln at this time fail to mention Crook. Lincoln’s escort was observed on several occasions by witnesses who provide enough detail to put names to descriptions. When the president met reporter Charles Coffin during his April 4 Richmond visit, the Boston Journal man helpfully identified those landing with the president, a list that does not include Crook’s name. So, there is, at present, no independent verification that Crook was there. Add to that a few small details that he has wrong. For instance, during Lincoln’s outbound voyage on the steamer River Queen, Crook has Lincoln declare he is "feeling splendidly" on the morning of March 24. Yet a review of actual messages sent by Lincoln and members of his party show that the president was not only seasick but likely also suffering from tainted water aboard the vessel. Crook’s post-war recollections are further muddied with claims counter to facts. He writes that both he and Tad Lincoln testified at the trial of John H. Surratt and indeed a review of the transcripts shows Tad as having appeared, but not Crook. There is no evidence he kept any diaries or notes of his time with Lincoln, so his extensive recreation of conversations held and overheard also strains credulity. In a masterful understatement, Lincoln scholar Wayne C. Temple noted: “Crook is a most unreliable source.”