First an online resource for you is Open Street Maps which you will probably find quite useful.
The main resource that I would suggest taking a look at is QGIS which is a suite of desktop applications that lets you import geographical data from various sources, (including Open Street Maps), select which layers of data you wish to display and at what scale, perform your own visualisations & add your own overlays and export to various image formats.
- Free, Gratis & Open Source
- Cross Platform, OS-X, Linux, Windows 7-10, (Android being worked on)
- Lots of plug-ins to extend functionality (651 at the time of writing)
- 1000s of data sources including demographic data for some contries, etc.
- Online tutorials
- the only danger is that you may get so hooked that you forget to write your book!
© OpenStreetMap contributors
Note on Licencing
Following discussion between B. Clay Shannon & Thomas Weller I felt that I should add some notes on Licencing to preserve the information.
There are basically 2 components that you need to check the licencing of for potential issues:
- The software QGIS - This is GNU GPL 2 - so if you include the software, in whole or part, in some other product, even if you modify it first, then your product will also have to have the GPL 2 licence but the output of running the program is not covered by GPL 2 so this does not impact your intended use - it is considered polite to mention that you used QGIS in the acknowledgements of publications produced in whole or part with it. (Non-lawyers summary)
- The licences on any data that you use: You must check the licence(s) for any data that you use in preparation of your maps - Open Streetmap has a very permissive licence that clearly states that you should provide the attribution, "© OpenStreetMap contributors", right next to any maps produced from its data and that that map, not the rest of the work, will be able to be copied but other GIS sources differ in their licences ranging from "attribution required" through "if you use this you owe us" to "you can't use this commercially in any way" so do please do check - preferably before downloading the data - especially from sites that require signing in before downloading as this is often a red flag that you might be entering into a legal agreement but not all data sources are that clear.
Please note that the above statements are true to the best of my knowledge, but I am not a legal expert, at the time of writing and may not be true in all legal frameworks, however for your stated purpose your publishers should be able to get a legal expert to check prior to publication.