Are you certain this is an eBook and not a PDF file? PDFs have no option for highlighting in iBooks. You can only highlight in ePub and iBooks Author books in iBooks. So try another book and see if this problem is consistent or isolated to a particular type of book.
To show your iBook’s controls, tap near the center of a page. Remember that not all books have the same controls, but some of the things you can do include searching, viewing the table of contents, and bookmarking the page. Only some books let you add notes and highlights. To add a highlight, touch and hold a word, then move your finger to draw the highlight. To add a note, double-tap a word to select it, move the grab points to adjust the selection, then tap Note or Highlight.
Are you using speak selection? If so, this often interferes with highlighting. Try turning off speak selection (settings > General > accessibility), quit iBooks, restart, and then, reopen iBooks. See if you get the highlight bar with all its functions except speak.
Hope something here helps!