How would I respond to a question regarding the textual and thematic analysis of the epigraph from Farewell to Arms, "this book is to be neither an accusation nor a confession, and least of all an adventure" ?

'This book is to be neither an accusation nor a confession, and least of all an adventure'
It is a good idea to pay attention to epigraphs and quotes which may precede the body of the text, as they often give clues as to the central concerns within the text, as is in this case.
The narrative voice (which is not necessarily the same as the author) is asserting the purpose of the text as they see it. The voice begins by challenging perceptions of war (note the number of negatives). You as the reader have the right to disagree as to the extent to which they have fulfilled the purpose they have set out.
Comment on the blunt tone and directness of the line, which is suggested by the explicit reference to "this book." Consider what is the effect of this? It seems to imply a sense of candour on the part of the voice. Consider the use of punctuation and its effect on the syntax. Hemingway uses several commas which reduce the pace. Is such a calm tone for such a dramatic subject matter as war unusual? Is it effective?
Now consider how this relates to Hemingway's style throughout the book. It is deceptively simple, with the effect of showing the brutality of the war which is often romanticised (comment on the word choice of 'adventure'). You may want to briefly refer to another part of the novel to back up your answer e.g. the visceral and unflinching description of the young soldier emptying his bowels during the shelling.
In a close-reading analysis it can be tempting to either focus too closely on the individual words and techniques, whilst forgetting about the bigger themes, and vice-versa. Starting with a close analysis can be helpful, and then considering why the author has decided to use these techniques (how do they help bring forth the main message/s of the text) can be a good way of making sure you keep the right balance.