"Ring of Fire" is a collection of short stories set in the universe created by Eric Flint in his 2000 novel "1632." In "1632," the town of Grantville is ripped from modern day West Virginia and dropped in the middle of Germany, in the middle of the Thirty Years' War, by a cosmic accident (commonly considered an Act of God) which is eventually dubbed the 'Ring of Fire.'
The fifteen stories in this anthology seem to have been written after the publication of "1632," but before that of its sequel "1633." They take place concurrently with the action in those two novels: "Power to the People" by Loren K. Jones goes all the way back to the Ring of Fire that kicked off 1632, while Eric Flint's "The Wallenstein Gambit" is set in the middle of the year 1633. As with all anthologies, the styles and qualities of the stories varied from author to author, from the 16-page "To Dye For" by Mercedes Lackey to Flint's 120-page novella. However, although I enjoyed some stories in "Ring of Fire" more than others, I can honestly say there were none I actively disliked, which is pretty much as good as it gets for anthologies.
What makes "Ring of Fire" so interesting is the fact that it is not your typical anthology. Most 'spin-off' anthologies like this one feature stories peripheral to the main plot of the series, involve minor characters and don't play a significant role in the grand scheme of things. Some stories such as "Power to the People" and "When the Chips Are Down" by Jonathan Cresswell and Scott Washburn seem to follow that model. However, most of them actually play important roles shaping both the plots and the characters of the later books in the series. For example, David Weber (who coauthored "1633") writes a story about the founding of the new American navy that plays a prominent role in "1633" and Andrew Dennis's story sets up "1634: The Galileo Affair" (which he coauthored) and develops some of its main characters. Meanwhile, "The Wallenstein Gambit" incorporates other stories in "Ring of Fire," redraws the map of Europe, and lays the basis for forthcoming "1634: ..." novels.
The point is that this anthology plays an important role in the series, and needs to be read by anyone who wants to enjoy future 163x books. This is a part of Eric Flint's interesting approach to the whole series, which tries to make the '1632 universe' a full-bodied and realistically complex place. To do this Flint writes the main books of the series with a number of different coauthors (David Weber, Andrew Dennis, Mike Spehar, Virginia DeMarce), while at the same time allowing all of the authors who contributed to "Ring of Fire" to make their own mark on the developing series. Flint has even begun publishing fan fiction in an online magazine (the "Grantville Gazette") and incorporating it into 163x novels. It is (to my knowledge) a unique approach, and so far seems to be producing excellent results.
So not only is "Ring of Fire" a solid anthology in its own right, it is required reading for those who intend to follow Flint et al.'s 163x series. Although "1633" builds off of several stories in "Ring of Fire," it can be understood and enjoyed without reading the anthology. The same is less true of the "1634: ..." books that are currently being written and published. If you enjoyed "1632" enough to want to read more books in the series, "Ring of Fire" should be a sure bet.
To conclude, and to emphasize one last time the important role played by "Ring of Fire" in the development of the 163x series, I will sketch out the current shape of the series and indicate which stories in "Ring of Fire" are incorporated into each of the planned books. Of the fifteen stories in the anthology, only three do not seem to play a role in the larger series, though they may yet be incorporated into one of the volumes currently being planned or written.
"1633" with David Weber
---"In the Navy" by David Weber
---"A Matter of Consultation" by S.L. Viehl
---"Skeletons" by Greg Donahue
"1634: The Galileo Affair" with Andrew Dennis
---"American Past Time" by Deann Allen and Mike Turner
---"To Dye For" by Mercedes Lackey
---"A Witch to Live" by Walt Boyes
---"Between the Armies" by Andrew Dennis
"1634: The Baltic War" with David Weber
"1634: Escape from the Tower" (Eric Flint solo)
"1634: The Austrian Princess" with Virginia DeMarce
---"Biting Time" by Virginia DeMarce
"1634: Bohemia" (tentative title) with Mike Spehar
---"A Lineman for the Country" by Dave Freer
---"The Three Rs" by Jody Dorsett
---"Here Comes Santa Claus" by K.D. Wentworth
---"The Wallenstein Gambit" by Eric Flint
"1635" with Virginia DeMarce
Stories that don't (yet) play a role in the series:
---"Power to the People" by Loren K. Jones
---"Family Faith" by Anette M. Pedersen
---"When the Chips are Down" by Jonathan Cresswell and Scott Washburn