Recent works by people like Thomas Piketty, e.g. his best-selling Capital in the Twenty-First Century also seem relevant.
IMO it’s good for a society if inequality is kept fairly limited. I suppose we have to mention Karl Marx too. There were some good intentions and a lot of interesting thinking there. I’m not sure, though, that so relentlessly viewing humans in terms of class and economic interests is so healthy. Some of the idealism also when badly wrong when put into practice. I suppose also that sometimes religion can indeed be used as “an opium of the people”! 
Piketty does set out a fairly plausible thesis though. Democracy does seem to have some ability to curb inequality. However, it may not be that strong. 19th century USA and Europe were very unequal places (the so-called “Gilded Age” in the US). Piketty argues that the easiest way to gain wealth in this period was to inherit it or marry into it (that the returns on capital in the long-term would be greater than build up from income or growth in the economy). It was rather a closed shop which tended to concentrate wealth.
Then there were the catastrophic wealth destroying (and levelling events) of the Great Depression, followed by the Second World War. I suppose the US then had what is considered one of its greatest Presidents, Franklin D. Roosevelt who (as well as his actions in the World War II) very skilfully put together a programme and political coalition of disparate political interests in his New Deal. I think in the long term this promoted a lot of economic equality in the US. In one form or another, this lasted until about the 1970s. In terms of wealth, there was a lot more equality in the decades after the war.
This held in Europe also. Another factor was the Cold War. I suppose the Allies and their governments looked nervously looked at the Soviet Union and feared various countries in Europe might elect communist parties. I think there were deliberate efforts to ensure a reasonable quality of life for citizens to try to prevent that (the Marshall Plan was probably not purely altruistic ).
The US also rather effectively kept check on corporate power in this period (companies that were too big got broken up through Anti-Trust Laws). Even from a free market perspective, breaking up strangling monopolies can be good for an economy. If corporations are allowed to have too much power, then this tends not to happen. Laws protecting workers were also reasonably strong even in the US.
However, this has started to gradually all change since about the 1980s (starting from Thatcher and Reagan). Inequality has increased in Europe and particularly the US since that period. Many of the remaining elements of the earlier New Deal have been undone (plus financial rules brought in after the crash that led to the Great Depression). Worker’s right have been weakened. In the US, effective income for the lower half of the workforce (those without college degrees) has basically been stagnant in real terms for about 30 years. Only the income of the top 2 or 3% has significantly increased (and by a lot too actually). Employment has gotten more precarious.
There have been darker sides to globalization. Benefits from that have flowed to the top few percent of income earners but not to most people (inequality within Western countries has been trending upwards). Democratic controls on multinational companies that span many countries, I suppose, are more difficult. There has been a tendency by governments to to cede powers to often remote international bodies. I think that reached its zenith in the negotations to TTIP . That deal would have resulted in quasi-courts allowing corporations o take national governments to court and fine them if not abiding by the agreement.
The US political system in recent decades does seem a lot more dominated by corporations (increasingly to an unhealthy extent). Some court cases have removed from restrictions on corporate donations.
I rather view Trump more as a symptom of some of these issues than a cause (or, even less likely, a solution 
).
I suppose what Piketty worries about is that this period of relative wealth equality was more an aberration than a norm for democracy/capitalism. He presented some evidence that in several respects we were heading more back in the direction of the 19th Century Gilded Age with increasing inequality.
Hopefully, that’s not the case. However, IMO Western society has been heading somewhat in that direction in recent decades. And even in purely economic terms, that may not be a good thing.
The whole interaction between political systems/democracy and capitalism/wealth/inequality is an interesting one. There seems to be, to some extent, an ability for voters to curb some of the worst aspects of capitalism. Then again when I look at some of the more extreme inequality in places like South America, I do wonder. Voters often appear to be quite happy to have (often the system probably encourages them to believe in ) a certain level of inequality (sometimes quite a substantial level too). However, the amount of power our current democratic systems give to people can be relatively limited (the ability to choose between two rather similar parties or politicians every few years).
Some guys with two dollars will suggest, even argue, with the seller that the price “should” be two dollars, or maybe three. (And some might then buy sandwiches, preferring to give them then have them sold at a price affordable to all.) Greed for wealth might not exist without greed for differential status. For some, I think, it’s only ok for everyone to have worthwhile work, if status is not equal; gotta have winners and losers.
