Heine's Reisebilder, published between 1826 and 1831, are part travelogue and part political pamphlet. The chapters on the Rhine — its castles, its boatmen, its peasant Catholicism and its English tourists — are funnier than most contemporary travel writing and helped shape every later Englishman's image of the gorge.
The same volumes include Heine's poem "Die Lorelei", the central myth of the Middle Rhine. Read it on the Koblenz–Mainz boat.