No dispute that Wilkinson's ideas deeply influenced all sane graphics frameworks thereafter. In fact SPSS' purchase of SYSTAT was (by far) the best thing they ever did, and that's why people still use that awful piece of garbage.
However, Hadley Wickham spent a lot of time thinking through which parts still made sense, and which didn't, in implementing ggplot2. That book is free online, as are Bostock's notes.
I heartily recommend all three, but if money is an issue you can get the latter two for a lot less than a copy of TGoG.
Funfact: Wilkinson started this by selling SYSTAT to SPSS, got bored, eventually went to Tableau (seems to have worked out nicely for them) and now is at h2o. As much as I despise SPSS from having taught classes that use it, I admire their business sense and wish I'd owned equity in them prior to IBM buying the company.
H2O might not be a bad investment either (I've been wrong plenty of times though).