Apps do not have to honor that action. You'll have to ask the apps author to fix or just find another app.
I have a Samsung Galaxy Note 10.1 and would appreciate some help. When I am viewing a downloaded book, it seems that I am unable to enlarge the text by using the 'Finger and Thumb' method or even adjusting the text size in the 'Display Settings' facility.
Many other applications allow the 'Finger and Thumb' method of enlarging the display, so why is this not available when viewing text in a book ?