I wouldn't rule out putting a glass sheet on top, but don't just dump it on the book - using spacers under the glass so the pages only touch at one point (technically a line) and adjusting the glass height and possibly angle should allow pages to curve and get you most of the way towards the effect I think you're looking for.

An alternative might be flexible plastic, adjusting the tension to allow pages to open. The problem with this approach is reflections are likely to be less predictable than glass, so lighting may be tricky.

A fine string (possibly fishing line) might also do the trick, and if you're not interested in shooting the whole book, this could be positioned out of shot - as could clamps, paperclips or light adhesive tape.

Another possibility might be to make whatever is holding the book part of the appeal of the image. Perhaps something like this. If you have a friend or model with attractive or interesting hands, you could show them holding the book as if reading it.

The problem here is the look I think you're after isn't natural for the book. We all expect books to fall open, but this can be an indication of damage. If you're shooting a book from a large print run and are intending to throw it away afterwards you could press it open at a particular point - that should give the look people expect - but you might have book lovers taking issue with that approach.