I read all 4 parts in the pre-released version on the interviewing.io discord. Here's my feedback:

Loved how much they go into the actual interview dynamics and phrases to say, where as other resources wave away interacting with the interviewer as an implementation detail. Haven't seen that anywhere else on the web and it's super helpful. The framework is also much more fleshed out than, for instance, the HiredInTech system design guide.

However, the 12 tech ideas section was a little too dumbed down for me, though it might be helpful for someone with less experience. I also noticed a few typos (MangoDB, cache vs hash) and told them, and they said they'll fix it in the next version of the guide.

3.5/4 stars; would read again