Doing an Introduction or a Preface or a Foreword - subtle distinctions - is much harder than a continuing historical or fictional narrative. It has to set up the story and "pre-qualify" the reader. I don't want anyone even trying to read my book who isn't a good fit for it - that is, who isn't historically literate and highly skeptical, especially of sacred cows such as the Enlightenment. And I want to give an accurate idea of what's contained and what great ideas may be contained. Tantalize and excite but don't give everything away - that's the idea!
Pictures - maybe some maps and graphs - would seem to be vital. My gosh, but a book is a lot of work. Lots less with the Internet, though.
Football's coming on TV today, but I'll still get some stuff done.