You can use
ILMerge to combine multiple .NET assemblies into a single .NET assembly.
You can use the approach from the following article (also there are some additional approaches mentioned at the bottom of the article under the 'Alternatives' section):
Load DLL From Embedded Resource[
^]
But because your library project is in the same solution what you can do is just edit your 'csproj' file so that on 'release' mode you include the compiling of the library project's classes as well.
That would look something like the following:
<ItemGroup Condition=" '$(Configuration)' == 'Release' ">
<Compile Include="relative-path-to-your-library-project\*.cs">
<Link>%(RecursiveDir)%(Filename)%(Extension)</Link>
<Visible>false</Visible>
</Compile>
</ItemGroup>