How is this quote ambiguous? "Truth in fact is the only quality at which I should have aimed, or did aim, in writing this book."
Something which is ambiguous is defined as having multiple interpretations or double meanings. The fact that More opens his text, Utopia, with letters explaining his intent illuminates that his identifies the idea that others can misinterpret the text and the ideas presented in the text.
Essentially, the ambiguous nature of the quote relies upon the fact that many truths do not exist as truths for everyone. Actually, a truth can only be considered a truth based upon one's personal interpretation of what is being stated or defined. If the individual fails to accept the "fact" as a truth, the truthfulness of the "fact" fails to exist. Therefore, the quote is ambiguous based upon the idea that not everyone hold the same truths to be factual. Also, More's inclusion of the quote allows readers to identify the text as being his personal truth, not the truth of the collective whole.