A map series is “a group of separate items related to one another by the fact that each item bears, in addition to its own title proper, a collective title applying to the group as a whole. The individual items may or may not be numbered” [Source = Cartographic Materials : a manual of interpretation for AACR2. American Library Association, Canadian Library Association, Chartered Institute of Library and Information Professionals, 2005, 2nd ed.]
Map series can cover anything from a town to the whole world, via administrative areas, physical features, regions, continents ...
The maps may be topographic maps (general-use maps at medium scales that present elevation (contour lines), hydrography, geographic place names, and a variety of cultural features) or show particular information such as geology, soils, land use ...
Map series may comprise a few sheets, or hundreds!
Sheets are usually published over a number of years, and some sheets may be revised before the first edition of others have been published
Since all have same specification they could be joined together to make 1 (very) big map
At CUL we catalogue map series at Series Level – i.e. there is one catalogue record per series (rather than one per map sheet)
When searching the catalogue for maps, this makes it even more important to search first under the most specific place you are interested in - a village, say - and then work up the geographical/administrative hierarchy - parish, county, country, continent ...
For each series we have a graphic index showing the layout of the sheets. (Our hard copy indexes are coloured to show our holdings ; there may be more than one edition of a sheet in the same classmark)
Non-series (monographic maps) are complete in their own right and each is given its own catalogue record
These books are invaluable reference works and provide brief histories of modern-era mapping for the areas concerned as well as graphic indexes for major map series. They will give you an idea of what mapping might be available to purchase or for consultation in libraries.
World Mapping Today by 
Inventory of World Topographic Mapping by A graphic index, or index diagram, is used to show how the sheets in a map series relate to each other.
When annotated, graphic indexes can also be used, for example, to show which sheets are available in an institution. They may also be used by publishers to show, for example, which sheets are published or in preparation, etc.
