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Re:CSS Grid Level 2 – subgrid is coming to Firefox by tlrobinson on Wednesday June 05, 2019 @05:14PM (#20109226)
I can recommend "The New CSS Layout" for getting up to speed on CSS Grid (among other modern and legacy layout techniques): https://abookapart.com/products/the-new-css-layout

It does mention subgrid too.

Re:CSS Grid Level 2 – subgrid is coming to Firefox by mmanfrin on Wednesday June 05, 2019 @09:56PM (#20110883)
Looks like the author of that is also the author of this post.
Re:CSS Grid Level 2 – subgrid is coming to Firefox by thirdsun on Thursday June 06, 2019 @02:30AM (#20112097)
You will find her name quite often when you research CSS grids - she's something of a layout and grid expert. If you'd like to read more, check out some of her fantastic posts at Smashing Magazine, where she's chief editor: https://www.smashingmagazine.com/author/rachel-andrew/