You could create a custom clip called "Yellow" with the following data:
Text: <span style="background-color:#FFFF66;">\1</span>
Then simply switch to code view, select Tools > Clips to open the Clips window select the Yellow entry and move the Clips window to the side.
Each time you find something you want to hightlight, click the Paste Clip button. This will apply a yellow background color style to the text that you selected, which is of course only visible in Book View mode.