The Callahan Series

Callahan's Crosstime Saloon: The short stories that introduced Callahan's bar, its regulars and regulations, and some of the walk-ins that continue to affect the place five books later. The rawest and most primordial of the series, but easily the most essential.
Time Travelers Strictly Cash: An anthology that includes four Callahan stories, along with a scattering of other stories, a book review, an essay and a speech. Excellent stuff both Callahan and non, although the Callahan stories in general are blown away by the brilliant "Serpents' Teeth."
Callahan's Secret: A plotline started in the first story of the first book comes to fruition with the end of Callahan's bar. Some very unexpected stuff here, and some material that by now seems commonplace.
The Callahan Touch: The narrator of the previous three books takes over where Callahan left off, and finds that even without the old man around, the bar still ain't in Kansas anymore, Toto. Unfortunately the weakest of the series to date.

Related Books

Callahan's Lady: Not actually a Callahan's bar book in its own right, this novel introduces Callahan's wife, Lady Sally, and gives readers a look at The Perfect Brothel and its newest (and affectively imperfect) employee. One of Robinson's most touching and human books in a career of touchingly humanitarian writing.
Lady Slings the Booze: This sequel to Callahan's Lady introduces a private detective with a phenomenal talent for deductive epiphanies. "Unbelievable" sums it all up nicely.