After Glow by Jayne Castle (Ghost Hunters #2 aka World of Harmony) on sale for $2.99 normally $7.99
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This one was originally published back in 2004. But her books (and Penguin titles in general) rarely go on sale. The metadata now lists it as book 2, but it was originally on her site as book 3 (as before goodreads, etc started the .5 listings for novellas). So some of the sites (like Kobo) still have it listed as #3.
It is the second full-length book in the series as well as the second full-length book with this heroine & hero, so not sure how well it would read as a stand-alone.
FWIW, the series did not start overlapping with the Arcane Society until book 5 and then some of the books were also part of trilogies, but even then I do think her books usually can be read as stand-alones as the overarching plot lines aren't as integrated as say the Psy/Changeling series. There's usually an on-going mystery or big bad organization they are fighting, but the individual mysteries are solved within one book. This is a rare case where it is just a continuation of the romance of Lydia and Emmett (their first book,
After Dark, did end with an HEA which is never in jeopardy here. They simply have a new mystery to solve.)