Np. Czytam sobie książkę v. Czytam książkę
Czytam książkę = I'm reading a book (neutral statement)
Czytam sobie książkę = I'm reading a book (for no particular reason and/or having a nice time doing so)
The exact semantics of sobie can be hard to pin down and be difficult to translate but often does have an idea something done for its own sake.
In the southern US (po)czytać sobie might be something like 'have myself a nice little read"