I'm studying C# 7.1 and .NET Core 2.0 - Modern Cross-Platform Development Third Edition written by Mark J. Price(by the way, great book), and I don't understand, as I wrote above, if I can use packages written for .NET Framework in .NET Standard 2.0.
Every help is appreciated, thanks.
Yes, you can, .NET Standard 2.0 supports .NET core 2.0 libraries as well as .NET framework libraries up to version 4.6.1.
For more info take a look at the table available on this page: https://learn.microsoft.com/en-us/dotnet/standard/net-standard.