Possibly Nightwings(1968), by Robert Silverberg.
Nightwings is the first in a trilogy of novellas, the next two being Perris Way (1968) and To Jorslem (1969). These three works were later collected into a single novel in three sections, also titled Nightwings.
In a decadent and caste-based future, humanity is divided into guilds,
each having a specific job to do. The members of some guilds appear to
have undergone genetic engineering, for instance, the Fliers' ability
to fly and the Watchers' ability to use their mental capabilities to
watch distant stars.
The main character in the novella is a Watcher whose mission is to
watch the skies with some sophisticated equipment and to inform the
Defenders in the event of an alien invasion. Along with a young Flier
girl and a Changeling (who belongs to no guild), he visits the old
city of Roum (suspected previously to be called Rome), and becomes
entangled in events including the possibility of invasion.
Apart from Roum, only two other great cities are mentioned, Jorslem
(Jerusalem) and Perris (Paris), but their greatness is relative, as
they only have a few thousand inhabitants.
Bit more Sci-Fi than fantasy, but it does have an item called an "overpocket" which
"...was infinitely capacious; the contents of a world, if need be, could
be stuffed into its shriveled gray maw, and still it would be no
longer than a man's hand. Gormon took from it bits of machinery,
reading spools, an angular thing of brown metal that might have been
an ancient tool, three squares of shining glass, five slips of
paper—paper!—and a host of other relics of antiquity."
~from Google Books' excerpt of the book.