Richard S and MartCross,
Thanks to both of you for your replies. MartCross is right - even East of ‘Italian’ (which is a standard dialect that nearly nobody speaks) there are Romance varieties that are nearer to Spanish. An example is Friulian which is spoken in Gorizia near the (potitical) border with Slovenia. It is important to not confuse linguistic borders with political ones, as you no doubt know due to your experience in Spain.
The intuition that there is a sort of “linguistic continuum” is not without its logic, however. It is frequently said that if you were a Catalan speaker and you were to travel from Barcelona to Paris (walking slowly on foot) you would understand people for a good portion of your voyage, whereas it you were to take the beginning and end points, you would have serious communication problems. In other words, if the continuum runs A to Z, A (Catalan) and Z (French) would be totally different languages, but A would have many similarities with B, C, D, etc. By about M, similarities would start to swing in the other direction - towards the Z (French) end of the spectrum. If you have ever studied Catalan, this will come as no surprise, as Catalan shares many similarities with French.
These are just anecdotes, however. For more substantial reading on the topic, I would recommend the following texts, which can be rather technical and/or abstract depending on how well you cope with historical linguistics. For better or worse, there isn’t much light reading on this topic:
The Romance Languages by Rebecca Posner
A History of the Spanish Language by Ralph Penny
This is not an exhaustive list by any stretch, but both books are quite readable, in my opinion. It may be of interest to you that Latin is just one part of a larger beast named Indo-European, which is the linguistic family Latin stems from. Most languages in Europe and Western Asia have Indo-European roots (Basque is an interesting exception). An interesting looking book on the topic is the following. I have not read it yet, but I would like to.
The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World by David W. Anthony
Another book which I hear is good is:
A Natural History of Latin by Tore Janson
It deals more with Latin and its enduring place in modernity, but it is supposed to treat the exportation and spread of Latin, which resulted in today’s Romance varieties.
Cheers,
Tim