I'm not a historian but I'm happy to suggest some articles that I've found useful in my own research on the subject. I work with current Haitian populations where the American Occupation holds a very contentious but important set of meanings for them. In particular, the debates surrounding the current UN occupation are linked to the American Occupation for groups that see both as illegal, immoral, violent, and depriving Haiti of its sovereignty. To better understand this perspective I did a little digging through journals and books so I can share my edited list.

• Fatton, Robert. The roots of Haitian despotism. Boulder: Lynne Rienner Publishers, 2007.

  Fatton's book is useful because it sets the occupation in the middle of a larger narrative about Haitian history. He makes the argument that despotism and authoritarianism in Haiti was reinforced through the American Occupation.

• Jean-Price Mars, the Haitian Elite and the American Occupation,1915-35

  This is a good look at the elite perspective of the occupation.

• Renda, Mary A. Taking Haiti: Military occupation and the culture of US imperialism, 1915-1940. Univ of North Carolina Press, 2001.

  This is specifically about the occupation and has a good overview of all of the problems involved. It also talks a little about the relationship of Vodou and the rebels as well as the American attitudes towards them. If you want more about Vodou and the state I can give suggestions there too!

• Bergman, Tabe Ritsert. "Polite Conquest?: The" New York Times" and" The Nation" on the American Occupation of Haiti." Journal of Haitian Studies (2011): 33-49.

  Good coverage of how different groups in America saw the occupation.

I can also give a short rundown of a few things but your questions are big and the answers complex!

America occupied Haiti largely for economic interests in Cuba and to curtail German influence in the region. One of the major ways they first took control was to wrestle control over the Banque Nationale d'Haïti. Whomever controlled the bank controlled Haiti so they used American banks and political influence to staff it entirely with Americans. Then, they withheld money from the Haitian government. No money, no government.

The Haitian government tried to resist by threatening to take the gold in the reserves of the bank. So US Marines landed, grabbed the gold, and took it back to New York. In 1915 Haitian President Sam was executed by Haitians after he executed 173 political prisoners. This violence gave America the pretense to officially occupy the country. I've never read a historian that argues this was done with altruistic goals in mind.

The US Marines did build roads and other public works. However, to accomplish this they reinstated corvée, which was technically legal under Haitian law but had not been utilized in a very long time. Corvée was conscripted labor that was used right after the revolution to get the nation on its feet. As you might imagine this created some mixed sentiments. Americans saw it is necessary for getting these projects completed. Haitians saw it as slave labor. It was a big rallying cry for the rebels who set up in the mountains and attacked throughout the occupation. I've read estimates that the US Marines killed as many as 11,000 Haitians during the occupation.

The US installed a handpicked president who they controlled, rewrote the constitution, and controlled the nation. This was largely unpopular in Haiti.

The American attitudes at home varied quite a lot during the occupation, though by the 1940s it was almost universally condemned by the popular press. The NAACP played an important role in bringing some of the issues to light for the general public. In 1920 James Weldon Johnson went to Haiti on behalf of the NAACP to investigate. You can read his report here but I'll quote some of it: (Some NSFW language, FYI)

The United States has absolutely failed in Haiti. It has failed to accomplish any results that justify its military Occupation of that country, and it has made it impossible for those results ever to be accomplished because of the distrust, bitterness and hatred which it has engendered in the Haitian people. Brutalities and atrocities on the part of American Marines have occurred with sufficient frequency to bring about deep resentment and terror on the part of the Haitian people. There have been needless killings of natives by marines. I was told that some marines had cut a notch in the stocks of their rifles for each native killed. Just before I left Port-au-Prince, an American marine caught a Haitian boy stealing sugar on the wharf, and instead of arresting him, he battered his brains out with the butt of his rifle.

I learned from the lips of American marines, themselves, of a number of cases of rape on Haitian women by marines. But, perhaps, the worst phase of American brutality in Haiti is, after all, not in the individual cases of cruelty, but in the American attitude. This attitude may be illustrated by a remark made by a marine officer at another time when I was seated at a table with some Americans. We were discussing the Haitian situation when he said, “The trouble with this business is that some of these people with a little money and education, think they are as good as we are.” The irony of his remark struck me quite forcible since I had already met a number of cultured Haitians in their homes.

The Americans have carried American prejudice to Haiti. Before their advent, there was no such thing in social circles as race prejudice. Social affairs were attended on the same footing by natives and white foreigners. The men in the American Occupation, when they first went down, also attended Haitian social affairs, but now they have set up their own social circle and established their own club to which no Haitian is invited, no matter what his social standing is. The Haitians now retaliate by never inviting Americans to their social affairs or their clubs. Of course, there are some semi-social affairs at which Haitians and Occupation officials meet, but there is a uniform rule among Haitian ladies not to dance with any American official.

A great deal of this prejudice has been brought about because the Administration has seen fit to send southern white men to Haiti. For instance, the man at the head of the customs service is a man who was formerly a parish clerk in Louisiana. The man who is second in charge of the customs service is a man who was formerly Deputy Collector of Customs at Pascagoula, Miss. The man who is Superintendent of Public Instruction was formerly a school teacher in Louisiana. It seems like a practical joke to send a man from Louisiana where they have not good schools even for white children down to Haiti to organize schools for black children. And the mere idea of white Mississippians going down to civilize Haitians and teach them law and order would be laughable except for the fact that the attempt is actually being made to put the idea into execution. These Southerners have found Haiti to be the veritable promised land of “jobs for deserving democrats”. Many of these men, both military and civilian officials, have moved their families to Haiti. In Port-au-Prince many of them live in fine villas. Many of them who could not keep a hired girl in the United States have a half-dozen servants. All of the civilian heads of departments have automobiles furnished at the expense of the Haitian Government. These automobiles seem to be used chiefly to take the women and children out for an airing each afternoon. It is interesting to see with what disdain, as they ride around, they look down upon the people who pay for the cars. It is also interesting to note that the Haitian officials and even the cabinet officers who are officially the superiors of these various heads have no cars. For example, the Louisiana superintendent has a car, but the Haitian Minister of Public Instruction has none. What the Washington Administration should have known was that in order to do anything worth while for Haiti, it was necessary to send men there who were able and willing to treat Negroes as men, and not because of their ability to speak poor French, or their knowledge of “handling niggers”. ...

The United States has failed in Haiti. It should get out as well and as quickly as it can and restore to the Haitian people their independence and sovereignty. The colored people of the United States should be interested in seeing that this is done, for Haiti is the one best chance that the Negro has in the world to prove that he is capable of the highest self-government. If Haiti should ultimately lose her independence, that one best chance will be lost.

This editorial really mobilized African American efforts to lobby Congress and force the US to leave Haiti. And eventually it worked, though the US also left because it had other issues on the horizon in the 1930s. After the US Marines left things were not particularly stable. The military force - Gendarmerie became the Garde - organized during the occupation was left to control the state but they learned from Americans a particular kind of enforcement that was incredibly violent. But because they were so central to the remaining government, leaders had to work directly with the military to maintain control. Power was also much more centralized in Port-au-Prince and class (along with racial) dynamics solidified. Historians like Fatton and Renda argue that this set the stage for despots like Papa Doc.