I've read both Richard Evans' The Coming of the Third Reich, and the first volume of Ian Kershaw's biography of Hitler, Hubris, but I still don't fully understand Hindenburg and Papen's decision to make Hitler chancellor. The National Socialists did well in the 1932 Weimar elections, winning 37% of the vote in the summer, and 32% of the vote later that year. However, neither of these outcomes allowed the Nazis to form a ruling coalition in the Reichstag. Hindenburg and Papen understood that Hitler was insane, and the former considered invoking Article 48, which would have given him emergency powers to basically rule as a dictator, and the Nazi threat would be averted. However, Goring told Hindenburg that, were he to invoke Article 48, this would lead to civil war. Schleicher, who was Minister of Defense at the time, told Hindenburg that the Reichswehr, being constrained by the Treaty of Versaille, was too weak to subdue the SA. There would be a destructive civil war, somewhat similar to the one that would take place in Spain later that decade, and the Weimar government would be unlikely to win. (I think the Reichswehr had something like 100,000 troops at the time, whereas there were about 300,000 Brownshirts, but maybe I'm wrong on that.) In any case, Article 48 was not an option. Then, for some reason, Schleicher becomes chancellor for two months, and maybe you can touch on that. But in the end, Papen basically says that Hitler can become chancellor as long as the vast majority of Hitler's cabinet — I think nine out of twelve — is staffed with "moderate" conservatives, while a mere three would be National Socialists. In addition, Papen would get to be vice chancellor. So Papen basically thought that he and the other cabinet members would control Hitler. Of course, we all know how that turned out. Can you explain why Hindenburg and Papen ultimately made the final decision to appoint Hitler as chancellor? Why not have the stability of Schleicher or Papen as chancellor, and not take a huge risk by putting a madman in charge? Did they "have" to put Hitler in power? What were the realistic alternatives?
The reason that Hitler gained power was by and large due to the instigation of Von Papen, it is important for us to first realize. The Junkers in general were essentially an anachronistic class of nobility that were very much in favor of having their power legitimized and continued. Von Papen, a Junker, had been essentially pressured out of office in the 1930's, due to the efforts of Hitler and Kurt von Shleicher, who replaced him as chancellor. Schleicher had come to power after severely pressuring von Papen to resign, and then was appointed chancellor by Hindenburg when von Papen in turn pressured Hindenburg to fire Schleicher. Hindenburg took Schleicher's side though partly on the condition that Schleicher maintain a public appearance of having no feud with von Papen, which Schleicher did not maintain and attacked von Papen in his public speeches a short time later.
The reason that Schleicher likely took a large degree of power was (A) that Von Papen was not popular, (B) while Nazis lost seats in 1932, Von Papen failed to secure a government capable of supporting him, and (C) Von Papen had backstabbed far too many of his potential supporters. Schleicher had wanted to form a conservative-center Nazi coalition and had originally appointed Von Papen to the government to do this, which is what gave Von Papen his own closeness to Hindenburg. However, Von Papen's backstabbing, leaving his original party, and his austerity policies basically killed any possibility for support. Furthermore, the Nazis would not support his government because he lifted bans on the SS and SA, which gave them no reason to be his ally at that point. So, von Papen resigned under pressures from Schleicher and others, seeing that his government would dissolve anyways, because he did not have the support to create a military dictatorship. Schleicher was then made chancellor by Hindenburg, as Schleicher still had his ear.
All of this eventually drove von Papen to become a puppet for Hitler. He ended up going into secret talks with Hitler to discuss tactics for removing Schleicher from power, which, because von Papen was still close to Hindenburg, had a strong upper hand. Eventually, the political battles ensued including intense lack of respect for Schleicher's government, and as a result an election was being prepared with Schleicher and Hitler as the front-runners, and von Papen agreed to mediate a government for Hitler and Hindenburg, with himself as vice chancellor. This was agreed upon by Hindenburg who allowed talks to enter into and negotiate. Schleicher in turn tried to court the Nazis for support of his government. Schleicher's government was grossly unpopular though, and he also made several critical missteps, like not allowing a long period recess in the Reichstag to take place, which, if it had happened, would have happened after his huge public works projects were finished and bolstered his name and worsened Nazi conditions greatly, who had refused to work with him. He did not do this.
A big problem also was that the Nazis were really only being challenged by the upcoming Communist party, and there was no negotiating between those two. The Communists fought for worker liberties, the dissolution of economic class oppression, the promotion of worker's councils, etc. and the Nazis were fascists and in bed with corporate capitalist ventures. As Robert O. Paxton noted, fascists and capitalists may not have always gotten along, but they made good bed mates. And Hindenburg, a conservative aristocratic president, did not want communists around.
So, in context, we have a political stalemate forming between Hitler and Schleicher, von Papen (an aristocratic Junker) is very close to Hindenburg (also from the aristocracy) and he hates Schleicher who took his old position and removed him from chancellorship, and the only real rival to Schleicher's government was Hitler. Hoping they could confine Hitler, they decided to opt with him, but giving Goring the Prussian Interior Minister office and Hitler the chancellorship was essentially a death's knell. In the end, Hindenburg likely saw a martial government led by the military as the only other option, given the ever increasing popularity of the Nazis. Von Papen was just only interested in his own ventures and did not care that Hitler planned to use the Enabling Act and dissolve the Reichstag in order to remove all civil liberties. Believing that Hitler would be reined in by von Papen, Hindenburg appointed him the chancellor, and from there... well it is all history.
I would suggest reading Peter Longerich's Hitler: A Life (Oxford UP, 2018).
Robert O. Paxton, The Anatomy of Fascism (Vintage, 2004).