Its Pronounced Tropay: Word Of God is that "Manticoran" is supposed to be pronounced with the emphasis on the third syllable (as "Man-ti-CORE-ahn"). However, all the audiobooks incorrectly put the emphasis on the second syllable (as "Man-TIK-er-ahn"). As it turns out, the fault lies not with the narrator but the author, who accidentally gave her the wrong pronunciation for the word in one of his emails. After the mistake was cleared up, it was decided that the incorrect pronunciation would stay in the sequels to avoid confusing the listeners.
Science Marches On: Impeller signatures can be detected FTL. Since a widely reported experiment in 2002 supported the idea that gravitational waves travel at light speed, War of Honor explains that this works thanks to an echo off of hyperspace and not via direct detection.
Shout-Out: Dr. Janet Frazier in At All Costs is an obvious nod to Dr. Janet Fraiser of Stargate SG-1. Weber confirms this in an interview and explains that he was upset when the character was killed off in the show and decided to bring her back.
What Could Have Been: The Honorverse was originally going to be timeskipped several decades, following the death of its namesake character. Her children would have continued the action. The Eric Flint collaboration Crown of Slaves nixed this original plan; its espionage plot ended up fast-forwarding the conflict by putting pressure on the Mesans to enact their plan early. As a happy side effect, Honor was spared, cutting off what would probably have been the greatest fan rebellion in modern Sci-Fi literature.
Word of Gay: Word of Weber is that Admiral Mark Sarnow is gay (and happily married), though the mention of it never makes its way onscreen. One forum poster did, however, note that it could potentially explain why Pavel Young never accused Honor of sleeping with him.
Write Who You Know: In A Beautiful Friendship, the Harringtons have a meeting at a restaurant called "The Red Letter", owned by a person by the name of Eric Flint. The fictional Flint is said to hail from the world of New Chicago, described as "a dumping ground for radical anarchists, socialists, and - especially - every member of the Levelers’ Association the government could round up after Old Earth’s Final War". The Real Life Flint, an occasional co-author with Weber in this and the 1632 series, is a self-admitted socialist, and currently lives near Chicago. Also qualifies as a Shout-Out.
Honor's actions in The Short Victorious War, assuming command of the fleet after her admiral is incapacitated, rather than having an admiral aboard another ship assume command during a key moment in the pitched battle, is likely based on the actions of US Navy Lieutenant Commander Bruce McCandless aboard USS San Francisco (CA-35) during the Naval Battle of Guadalcanal. Like Honor, Commander McCandless was decorated for his actions rather than court martialled (he was the seniormost surviving officer aboard San Francisco, but by far not the seniormost officer in the task force), probably in large part because the outcome was an American victory against a larger Japanese force.