That reminds me---there is an obscure book by Esther Friesner called Druid's Blood that might appeal to Merlin fans. It's not directly Arthurian: it's an alternate-history fantasy in which the Druids used their magic to repel the Roman invasion...and every invasion since then.

Fast-forward to the reign of an analogue to Queen Victoria, ruling a still-quite-Celtic Britain by right of heritage plus ability to wield druidic magic. Suppose there lives in her magic-ruled kingdom an analogue to Sherlock Holmes, solving mysteries by sheer logic, accompanied by his friend the humble country doctor. And structure it as a story of power struggles, loyalties and betrayals, and chapter titles building off Holmes' canonical adventures but leading to entirely original adventures.

I would rate it PG for one scene of non-explicit hetsex. And it has secrets and lies, oh yes.