In this book, Forster is very intentional in defending Calvinism not by systematic scripture proof, but more by philosophical explanation. So this is helpful for those who have thought carefully about Calvinism and its implications, but it’s definitely not a primer for those merely exploring the doctrines of grace.
For example, in defending limited atonement, Forster explains that the non-Calvinist position requires one to believe that Jesus died for "humanity" merely in the abstract, but not for individuals personally (52), because if Jesus truly substituted himself on the cross for every individual, then every individual would be saved. Since we know that not every individual is saved, the non-Calvinist must depersonalize the atonement as something targeted only for humanity, and not for individuals. "If Jesus's work only makes salvation available, then it cannot be substitutionary in any meaningful sense." (56).
I think this is a profound and helpful observation, but it might not connect with readers who don't know what bothers people so much about limited atonement.
If you want a simple explanation of Calvinism, there are many better options (such as John Piper's book, "Five Points"), and I think Forster would agree. But if you have studied Calvinism, and it just doesn't add up to you, this book could be very helpful.