3. The books based on the shows other than TOS tend to be set after the series because there's more storytelling freedom, since characters can grow and change. Particularly for something like DS9, which was heavily serialized, there's not as much room to fit in stories during the series. But TOS was always a more episodic series and so standalone stories that put everything back in the box at the end seem to be better-received by TOS fans.
Plus, the later series were left open-ended so there's more future to explore, as opposed to the TOS characters whose futures were roughly charted over the course of the movies--and where we already know the future of the Federation up to the TNG era.
Granted, there are still nooks and crannies in the 23rd Century left to explore, but it's not like the 24th Century where you can keep on going without bumping into the pre-established continuity of the Next Next Generation!