A bookmark doesn't have any text - it's just a page marker. You can also bookmark a page while reading by double-tapping the upper right corner of the page.
Are you sure that you lose lines when you display the notes menu? You don't on the PRS-600 (which has pretty much the same firmware as the 900); on the 600, when you turn on the notes menu, the text on the page gets slightly smaller to accommodate it, but displays the same number of lines.
Currently proofreading The Poison Belt, by Sir Arthur Conan Doyle.