If you copy their pattern, you have violated copyright law. However, if they copied the pattern from the public domain in the first place, you're fine. If you start from their pattern and change it a little, you have created a "derivative work" of the original pattern and have still violated CR laws.
If you create your own pattern and it just happens to look like someone else's pattern, you have not copied or violated CR laws. Instead, you have independently created a matching work.
Read the pattern books and see if they declare their patterns copyrighted or if they restrict how you can use the patterns.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.