I bought the official cert guide, which, although the tone of the book is readable and conversational, I found that it covered only about 25% of the questions in the foundations exam. I was really surprised how off the mark it was, I actually sat Foundations 3 times because I was totally out of my depth with a lot of the questions - but maybe you'll understand it better given you're experience - I can send you my study guide if you like. But I found that the VMware KB and whitepapers were the best resource and 'Mastering vSphere 6.7' book helped too.
