Travel writing or a travel book is a genre which is
1) part memoir, a piece of autobiography, story of a person's life during that span of time when he (or she) was away from the place he considered home;
2) imaginative description and fantasy based on information culled from books, the experience of the place, people met and talked to while there, a place foreign to him in some way, which is presumed to be foreign to the imagined average reader;
3) not a guide book for it is not written on the premise the reader is going to visit the place but rather on the premise she (or he) is not;
4) the imaginative recreation of the place (armchair travelling) is combined with an examination of the culture through fresh eyes as the same time as the author re-examines his own culture.
It may be written in journal form, as a diary, as an essay, as an extended narrative the chapters of which are organized around stopping points or themes or turning points in the author's life while at the place.
Further comments: it is a form of fiction--(while fiction as partly a form of disguised autobiography)--that is by no means are we told everything about the author's experience, and that which we are told is shaped to be dramatic or lyrical. It is at the same time highly autobiographical, very personal, and therefore does not fit into the modern classical canon of fiction, poetry, and plays, or the older one of which moved from the older classical canon of tragedy to comedy to history writing to essays and letters so does not yield itself to the usual tools of analysis (characters, plot, themes, interlace, point of view, irony &c) that we find people endlessly applying to novels and plays and to those poems which tell a story.
It forces us to be original in our approach because it is a highly original and free form of writing.
I have in mind not only the above but many other books.
Some examples: there are those in which the travel stays put once he gets there (like Durrell), e.g, Gerald Brenan's South from Granada; there are those in which the traveller gives a great deal of history and information, e.g., Robert Byron's trip through Afghanistan and southern or Byzantine-influenced Russia, Road to Oxiana which becomes a treatise on architecture; there is the deeply imaginative meditation which tells us nothing of the author's private life, e.g., Eleanor Clark's Rome and A [Hadrian's] Villa; and of course the hundreds which dwell on the author's life very intimately and give us moments of self-discovery in a foreign land, Robert Louis Stevenson's Travels on a Donkey in the Cevennes.