Though dated (and broken into two parts, this being only part one), this monumental and magisterial history of the Jews in Christian Spain remains your first stop in any bibliographical reading on the topic. There's been more debate since the book came out in 1949, and was translated into English in 1961, about the actual inevitability of the expulsion in 1492, among other things (Baer's approach is strongly influenced by the Holocaust, which ended only a few years before the book came out). One could also debate Baer's clear historiographical bias toward Castile when there's more material to be had from Aragon (then again, my concentration is in Aragon, so I freely admit to wishing he'd focused more on that region). Plus, his coverage of women in medieval Spanish Judaism is pretty thin.
That said, this is the book you need to start with in your reading because this is the one everyone else after him refers to, with good reason. A huge amount of primary research went into this tome. The endnotes are worth the price of the book, alone. As for readability, the English translation is actually quite narratival and therefore, something a lay person could enjoy without knowing a whole lot on the subject, I think.