If I read that correctly, you are schooling a 5 year old- right;
Out of curiosity, may I ask why you bought the Grade 2 study and not Grade 1?
I assume that you have gone through the learning library for the how to teach Charlotte Mason style using the SCM resources. I am a 8th year homeschooler and just moved to a full Charlotte Mason schooler in the last 3 years and I keep learning more 🙂
To answer your book question, you will get new books for each year of history and you can use the science guides over and over if you want for nature study, or use those two years in a row and then use something new- or do Birds one year, Insects one year, Outdoor secrets etc.
When your younger kids hit the “school age” of 6ish they can join in with the lessons as a family and only need their own math and language so those books will be bought for each kid, but the history and science (up up middle school) can all be read together.
I do buy most of our books for the year but I also get a few at the library because we only read them for a week. Biographies we buy.