explicate (the cognate of explicar) to mean 'conjecture or explain possible reasons or origins', rather than books like Breaking Out of Beginner's Spanish and 1001 Pitfalls in Spanish which only state the difficulties with no explication whatsoever.
What books explicate Spanish's difficulties, especially those counterintuitive to an Anglophone or Francophone? I exemplify my meaning of 'difficulties' below, which no grammar book have explicated; I found the explications on Wikipedia's page about 'Vulgar Latin'.
[1.] [...] Generally, the personal pronoun is unreduced (beyond normal sound change), while the article has suffered various amounts of reduction, e.g. Spanish ella "she" < illa vs. la "the (fem.)" < -la < illa.
The book must explicate such connections between words (or Functional Morphemes).
[2.] Most languages have one main copula, but some, such as Spanish and some other Romance languages, have more than one. This is because the verb or verbs meaning "to be" in the Romance languages are derived from not just one but THREE Latin verbs [...]
1 introduced me to 2. Unhelpfully, grammar books neglect Latin copulas (sedēre, esse, stāre), which otherwise would easily aid learners to disambiguate ser vs estar, partly because:
2.1. they both are more specific than 'to be' and 'être'; though a cognate of estar, 'être' is broader because it subsumes the meanings of ser.
2.2. knowledge of the Romance Copula explicates the irregularity of the conjugation of être.