Please advise how https://linguistics.stackexchange.com/q/13163/5306 and https://linguistics.stackexchange.com/q/13162/5306 can be made less broad, for reopening, please?
For the first question, it is also unclear what you want to know. Are you asking for the best introduction to Spanish historical linguistics? There might be 3-4 candidate books, so it would be useful to explain in more detail what kind of book you're looking for (also, whether it matters to you what language the book is written in). By analogy, a question asking for references on Indo-European historical linguistics would generate a huge number of replies. Narrowing it down to "the best introduction" would help -- one might consider Outline of the Comparative Grammar of the Indo-Germanic Languages, or Proto-Indo-European Phonology and Proto-Indo-European Syntax, or Indo-European Linguistics: An Introduction, or Introduction to Indo-European Linguistics, but probably not New Comparative Grammar of Greek and Latin or Introduction to the Laryngeal theory. But Outline of the Comparative Grammar... is more of an exhaustive compendium and might be overwhelming if you weren't already familiar with the subject, so it would depend on whether you were looking for a rigorous introduction vs. an elementary introduction. We have no idea what you already know.
I don't understand the second question at all. French doesn't have any difficulties, so there's nothing to explain as far as I can see.