It was blank at the beginning of THE AIR PLAN. I right clicked and selected from imported clips, the <p class="capt">. I went to the end of JUNE 1944 and used control period to insert a </p>. Then I clicked on book view and got the red screen message. I went back to code view and saw that an extra </p> was added just before </body>. I deleted it and then it went back to book view without any further problem.
It may not be a consistent thing. I have not encountered it until this morning since the last time.