to indulge in; to give oneself up to; to be obsessed by; to be engrossed in; to be lost in; to be absorbed in
to be absorbed in reading; to get lost in reading; to be immersed in (a book)
to be absorbed in thought; to muse; to muse about; to think things through
to be lost in thought; to be immersed in reverie
be absorbed in deep thought; be sunk in thought