Really, that was the idea of the first few TP books -- to be in the vein of old crossovers like Invasion! or Section 31, a group of standalone adventures in the various series that were loosely linked by a common element or theme. One is unambiguously TTN, one novel (and one e-novella) is unambiguously TNG, one is unambiguously DS9, and only one is kind of a hybrid that's hard to pin down. It's only the more recent Typhon Pact books that are full-on multi-series crossovers.
Even there, I would argue that Plagues of Night/Raise the Dawn
are really Deep Space Nine
novels. Yeah, the Enterprise
crew appears, but they're mostly guest stars -- the Deep Space 9 crew is the center around which those novels pivot.