Standard itself is used also, as a noun. OED definition of this sense:
A book generally accepted as an authority, or regarded as an exemplar of excellence.
Oxford English Dictionary, s.v. “standard (n.), sense III.22.a,” June 2024,
https://doi.org/10.1093/OED/1070004760.
I've been recommended C# in Depth as the standard on C#.
Gold standard is a viable option as a phrase also. The modifier 'gold' can serve to intensify the meaning.
I've been recommended C# in Depth as the gold standard on C#.
'Gold standard' is typically used in the sense of a 'benchmark.' While it can denote the best of its kind, it’s usually referenced as a point of comparison or measurement against other similar items. For instance, saying 'C# in Depth is the gold standard for C# reference books' could work better, though the term is not commonly applied to books. In this context, 'standard' alone would be an adequate fit for the OP's sentence.
Wiktionary has this example also (credit to Mari-Lou A) :
The OED is the gold standard for English dictionaries.
Additionally, standard is used in the same sense in its adjective form:
Of a book or an author: generally accepted as an authority, or regarded as an exemplar of excellence.
Oxford English Dictionary, s.v. “standard (adj.), sense I.3.a,” June 2024, https://doi.org/10.1093/OED/6735049595.