$49 is nothing.
Well... in the long term $49 is nothing. I agree that these days I'll shell out $50 for a book without much thought. However, the OP is a teenager who may not have access to much money. In the short term, if $49 is too much for you, then learn from other sources.
At your age (17 or 18) and pre-college status, there is not much you can do over two months that will radically transform your knowledge and ability.
If you want to study a language or learn a tool or technology, then go for it. But for the next several years you will be studying a wide range of topics in depth. Two months of your own undirected study is less than 5% of what you should get out of a 4 year degree. Sure, it is greater than zero, but be realistic at how much it actually is.
I encourage you to find some hobby projects you can build while in school beyond your class projects. If you can find ways to use the knowledge in your classes then great, but building some projects outside of class is a great way to build a useful portfolio and to gain experience.