On April 12th, Apple began notifying customers running macOS High Sierra 10.13.4, about apps that have not been updated to use 64-bit processes. 32-bit apps now produce a one-time alert when opened. This notification gives users notice that they are running 32-bit software, which will not be compatible with macOS in the future.
VitalSource is working on a 64-bit version of Bookshelf, version 8.0, that is due out in the summer of 2018. You may continue to use the 32-bit version of Bookshelf, version 7.5, on your Mac today. Using 32-bit software has no adverse effects on your data or your computer.
Visit Apple’s Support Page for more information on Apple’s timeline for transitioning macOS X software from 32-bit to 64-bit.