Stiletto & Oxford Reading Order
(and addressing the pesky question of whether it's one series or two!)
The Stiletto and Oxford books are technically two series, and the publisher of that series defines them as such.
However ... I myself think of it as one series. Stiletto came first, envisioned as three-book series. A fourth book Stiletto book was later added, which spawned a 4-book spin-off series (Oxford).
Together that makes 8 books, all set in the same world (office building), and it's worth noting that out of the 8, six of the books feature Oxford heroes. So you see. It really is more one series than two ;-)
They're all stand-alones, and can be read in any order, but I know plenty of you prefer to read them in proper order. I'm one of those readers myself ;-)
The full series, with covers
(note: The first four Stiletto books got new covers in the Summer of 2017 to better complement the Oxford covers, but the book content is the same as what was released in 2013-2014)