A Short History of Thomism, by Fr. Romanus Cessario, OP, is perfectly described by its title. It runs to a mere 96 pages, justifying the "Short," and its ten-page index suggests the density with which the history of Thomism is covered.
In the conclusion, Fr. Cessario writes:
In the brief space that a study of this kind allows, it has been my intention to stress the real, dare I say personal, unity that binds all these diverse members of the Thomist school. By examining, even in a brief and cursory way, the work of past and present Thomists, their manuscripts and printed volumes, their translations, commentaries, and compendia, I have endeavored to establish clearly that Thomism is not an abstraction, but an active force that has shaped the minds of clerics as well as of lay and religious scholars in a most personal way.
The book is certainly brief and cursory. While it mentions the various debates between Thomists and Scotists, Molinists, etc., it will cover the point of contention -- which in some cases practically consumed entire careers -- in a page or less, and if you want to learn much more about any particular Thomist than when he lived and the name of his most important works, you'll probably have to look elsewhere. As for the various schools of Thomism that are around today, they're mentioned, but again not in enough detail to really understand the differences involved.
The question, "What is Thomism?" is not easily answered, though a couple of passages do give, not categorical definitions, but descriptions of Thomism; I may get around to quoting those passages. For now, I'll finish with the final words of the book, which offer a suggestion of why the question might be worth asking:
Thomism, we know, centers the searching mind on God, from whom all blessings flow, and then moves to capture the searcher's heart. The supreme blessing that first drew the attention of Aquinas is the mystery before which he knelt each day, the blessed gift of the hidden Godhead, which under the figures of bread and wine held Aquinas captive to his Lord, and which today continues to sustain those Thomists who want to enter into his thought with the most perfect assurance.