Hi Andrew,
I've only just begun looking into Smalltalk/Squeak in the last few months and found that there are quite a few books available from the list shown in the link below
https://squeak.org/documentation/
and they are almost all available for low cost from used bookstores (or Amazon used books) that have been helpful. Many of them are dated in terms of the Squeak version they use but I've found them helpful regardless. I'm about halfway through Squeak: A Quick Trip to ObjectLand that I paid $7 for and was in perfect condition. It is sort of a cutsie approach to Squeak and Smalltalk but it's actually very effective in showing you the basics of the Squeak environment as well as the Smalltalk language. If you can afford it I'd recommend going on Amazon or Abe Books (used bookseller) and trying to find books availble from the link above. I think i've spent < $100 and have about 8 or 10 books on Smalltalk or Squeak and they all seem very well written so far. It's been fun reading through these books especially for someone that grew up during the heyday of Smalltalk but never learned the language. The new Squeak By Example 6.0 version is also excellent and helpful in getting up to speed which is a free download from the above page as well.
Good luck!