If you want a little starter book then go for this...
https://www.amazon.com/See-Say-Spanish-Word-Picture/dp/0451168372
This is her book for serious Spanish learners...
https://www.amazon.com/Madrigals-Magic-Key-Spanish-Creative/dp/0385410956
It's been in print since 1953 because it works.
>Committed to learning Castilian Spanish well in the next 4 years. I’d like to get into those little starter books recommended but I would love a weekly lesson type deal with vocab and stuff.
And you'll never learn Spanish well in 4 years by doing weekly lessons.
It has to be virtually every day.
I've been studying Spanish for over two years on my own and I just this summer learned how to conjugate virtually any verb in Spanish. (Regular verbs are easy to learn. You can put the rules for that on one page. It's all the irregularities that make it complicated.)