Root Lenovo Yoga Book YB1-X90F

One must first downgrade the Android OS to Nougat following the instructions here.
Be patient between steps as there is a slight lag between screen prompts.

After downgrading to Nougat through Intel Platform Flash Tool Lite., enable Developer Options by tapping Settings | About | Build Number until Developer options is confirmed.

Under Developer Options turn on OEM Unlocking and USB Debugging while turning off Automatic System Updates.

Connect Yoga Book to PC with ADB and Fastboot already installed and the environment variables set so that one can open a command line and issue ADB and Fastboot commands.

Open the Windows Command Prompt and type "adb devices".

If the Yoga Book is listed, then type "adb reboot-bootloader". If the Yoga Book is not listed, you must figure that out on your own.

Once the device is showing the boot loader, type in the command prompt "fastboot flashing unlock"

If that does not work, then you will likely have to use the adb and fastboot commands that are in the directory c:\Users\[your user name]\AppData\Local\Android\Sdk\platform-tools, so open a command line or power shell while in that directory.

After a successful unlock and reboot, you will encounter the set up process again. Complete the set up and enable Developer Options again with USB debugging.

Now you must download TWRP Recovery image to the same directory as Fastboot.

Download all the necessary root files from here aka MMRoot.

With the Yoga Book connected to the PC, copy the MMRoot files from the PC to the Downloads directory on the Yoga Book.

Open a Windows command line and type "adb reboot-bootloader", then "fastboot boot recovery_twrp.img".

Use the TWRP menu to Install 1-Magisk-v7.zip

Then Install 2-phh-superuser-magisk-r259.zip

Links that helped compile these steps

Yoga Book Nougat Stock ROM
XDA root Yoga Book
XDA root Yoga Book revised