The world's most acclaimed travel writer takes us on one last African journey, from Cape Town to Angola.      "Happy again, back in the kingdom of light," writes Paul Theroux as he sets out on a new journey though the continent he knows and loves best.      Having travelled down the right-hand side of Africa in Dark Star Safari, he sets out this time from Cape Town, heading northwards in a new direction, through South Africa and Namibia, to Botswana, then on into Angola, heading for the Congo, in search of the end of the line. Leaving the Cape Town townships, traversing the Namibian bush, passing the browsing cattle of the great sunbaked heartland of the savannah, Theroux crosses 'the Red Line' into a different world: one increasingly removed from both the intineraries of tourists and the hopes of post-colonial independence movements, 'the improvised, slapped-together Africa of tumbled fences and cooking fires, of mud and thatch', of heat and poverty, mobs and anarchy.     A final African adventure from the writer whose gimlet eye and effortless prose have brought the world to generations of readers, The Last Train to Zona Verde is Paul Theroux's ultimate safari.