As for the causes, to boil it down to super simple terms and not make this a 30 page essay, it was a mix of anxiety from the French and especially Napoleon III and the political maneuvering of Chancellor Bismarck and the growing power of Prussia. Following the Austro-Prussian war of 1866, a large part of Germany was unified under Prussia who had formed the North German Confederation. Now, most people in Europe believed that Austria was going to win, so when Prussia came out victorious and unified the majority of Germany in a confederation led by them, it caused massive panic. This is especially true in France. France at the time was kind of seen as the premier power in Europe. While Britain was stronger economically than France, it kept mostly to its colonies and was not too involved with continental politics and power plays. France on the other hand, wanted to be the predominant power in Europe and its military was regarded as the one of if not the best in the world. A unified Germany, which housed a large population as well as a large industrial base, threatened this standing. France had made threats to get involved in 1866 and back Austria but the war was over before they could act on anything.
The Franco-Prussian war began in 1870, so from 1866 to 1870 there began this political battle of wits between France and Prussia. Napoleon III, now an aging emperor and in rather power health, was desperate to gain back the power and popularity he once had and secure his dynasty for his son. This led to him making rather risky power plays to try and show strength to both the French people and also Prussia. Napoleon III's attempts to receive territorial compensation of Luxembourg to help offset the power Prussia gained in 1866 failed. Many view a war between the two nations as almost inevitable, and many within Prussia leadership such as Bismarck saw it as tantamount to securing German unification. Although he also claimed to want peace and not war, this was more than likely political maneuvering to make France and Napoleon III seem the aggressor.
The biggest issue that led to Napoleon declaring war against Prussia and the North German confederation was the crisis of the candidacy for the Spanish throne. The revolutions of 1868 in Spain left the throne vacant, and one of the potential candidates for the throne was German Prince Leopold von Hohenzollern-Sigmaringen. Leopold was married to the daughter of King Ferdinand II of Portugal meaning upon Ferdinand's death, the two kingdoms could be unified. A German ran kingdom to the south of France scared Napoleon III as he would be sandwiched between two German ran kingdoms, putting France at a disadvantage. Napoleon III demanded King Wilhelm disallow Leopold from claiming the throne. This is interesting as Leopold was related to Napoleon via the Murat-Bonaparte lineage as well as he had endorsed his Leopold's brother Charles and his successful bid to the Romanian throne in 1866 and recommended Leopold to the Greek throne in 1863. It was the political situation after 1866 with Prussia that changed his opinion. Both King Wilhelm and Bismarck were unaware of Leopold's candidacy, however Bismarck did not necessarily think it was a bad thing either.
French ambassador to Berlin Count Benedetti and King Wilhelm had a discussion on the matter and King Wilhelm asked Bismarck to create a report of the interaction. This became known as the Ems telegram, in which Bismarck made alterations to the events that occured. His alterations made Benedetti more demanding and the King more abrupt. To the French, it read that the King had insulted Benedetti and to the German people it appeared as Benedetti offended the King. After this event, Napoleon III pushed for a declaration of war against Prussia and the North German Confederation.
The aftermath is that a unified German Kingdom was formed, with King Wilhelm becoming Kaiser Wilhelm I. France was politically embarrassed, the Second Empire collapsed and the Third Republic was formed. The territories of Alsace-Lorraine were given to Germany and they became the predominant military power on the continent. Military systems all around the world before the war were based on the French system, however after the war nations began to follow the German military style such as tactics and the development of a dedicated and efficient General staff.
Some good books I can recommend for the topic are:Howard, Michael. The Franco-Prussian War.2nd ed. New York, NY: Routledge, 2001.
Wawro, Geoffrey. The Franco-Prussian War: The German Conquest of France in 1870-1871. New York, NY: Cambridge University Press, 2003
Wetzel, David. A Duel of Giants: Bismarck, Napoleon III, and the Origins of the Franco-Prussian War. Madison, WI: Univ. of Wisconsin Press, 2001.
Showalter, Dennis, The German War of Unification.2nd Ed. New York, NY: Bloomsbury Academic, 2015Stone, David. First Reich: Inside the German Army During the War with France 1870-1871. London: Brassey, 2002
Bresler, Fenton. Napoleon III: A Life. New York, NY: Carroll and Graf, 2000.
Ascoli, David. A Day of Battle: Mars-La-Tour, 16 August 1870. Edinburgh: Birlinn, 2009.
Barry, Quintin. The Franco-Prussian War 1870-71: The Campaign of Sedan. Vol 1. West Midland, England: Helion & Company, 2007
Both Micheal Howard and Geoffrey Wawro's books are probably the best books out the subject and cover the wars entirety without getting bogged down in every detail and are readily available online and at a lot of books stores.