There have been numerous firmware updates to the Surface Book for sleep/wake issues
since its release. These have resolved the issue for most people but not for some,
among which may be your Surface.
The post
"Sleep of Death" on Surface Book: What the Problem Is and How Microsoft Should Resolve It
has explanations and advice on preventing the sleep-state that Microsoft has
invented for the Surface, called "Modern Standby" and also known as "S0ix".
The procedure used by the author to solve his problem might also work
for you:
Turn on hibernation, by running in an elevated Command Prompt
the following command:
powercfg -h on
In Power Options, use the Change Plan Settings button and then:
For both On battery and Plugged in, set "Turn Off the Display" and
"Put the Computer to Sleep" to "Never", then click Save changes.
Click Change Advanced Power Settings and under Power Buttons and lid,
set all Lid close, Power button, and Sleep button options to Hibernate.
Back in Power Options, click Choose What the power buttons do,
click Change settings that are currently unavailable and disable
both "Sleep" and "Turn on fast startup".
Instead of turning off the computer screen when idle, use an economical
screensaver.
You will find more information and explanations of these in the linked article.