No, he is not a must read as if he speaks for the magisterium of the Church or the Catechism of the Catholic Church. He is associated with the Nouvelle Théologie (New Theology) movement that emerged in the 20th century among various catholic theologians who were more or less critical of scholasticism and Thomism which has been the dominant theology and philosophy in the Church for many centuries. St Thomas Aquinas is one of the greatest doctors in the Church and many popes have sung high praises of him and his writings even the Second Vatican Council as the Church’s foremost teacher in theology and philosophy studies. As one pope has said, one will not stray from the truth or the faith taking St Thomas as guide.
The New Theology movement is associated with what I call ‘new age’ philosophy and various modern philosophical systems that have emerged since the Enlightenment or modern times. Personally, I find no interest in these systems of which some I believe are simply incompatible with the Catholic faith and the works of some of these Nouvelle Théologie catholic theologians from what I have briefly read can be quite unorthodox, ambiguous, and either borders on heresy or is heretical and dangerous to the faith. Its been termed a progressive or modernist theological movement. Fr. Reginald Garrigou-Lagrange, a well known domincan Thomist theologian in the first half of the 20th century, was critical of the movement and the "New Theology’. From what I understand or have read briefly is that this ‘New Theology’ is in some ways relativistic towards the Church’s perennial doctrine and teaching.
Karl Rahner is known to have been quite learned and exercised considerable influence in catholic ‘modern theology’ in the Church in the last century. From what I understand, it is up to debate whether some of his writings and thought is orthodox or unorthodox depending on how one interprets him. I personally have not studied or read much of his work because, frankly, I have no interest in it. I am a Thomist at heart. I do have a work of his called the Encyclopedia of Theology - The Concise Sacramentum Mundi which at times I look at concerning certain questions of theology but which I haven’t looked at in some time. From what I remember, he gives a pretty good historical analysis from the Church’s theological tradition and decrees from ecumenical councils and popes concerning various theological doctrines of the Church but I didn’t always agree with his own opinion and conclusion. Some of it is difficult to read due to his adherence to what I call ‘new age’ philosophy such as Transcedentalism, Phenomenalism, or whatever it is called and these philosophies, as I have said, I have no interest in.