(I am currently reading Billiards at Half-Past Nine. I thought that Silent Angel was good, I guess, when I read it - because the topic was of interest to me. But now I see it is merely juvenalia and have lowered my rating (from 4) to 3. Nothing in it prepared me for the mature Böll, which is now overwhelming me.)
Written in 1950, this book was not published until 1992 - 7 years after Heinrich Böll's death. Apparently the nation could not cope with a book of this sort. The reasons for this are well explained by Sebald, who discusses the 'cultural amnesia' that has afflicted Germany since the end of WWII - (in the Natural HIstory of Destruction). At any rate, this is one of the better examples of what is known as Trümmerliteratur -- much of which, to listen to Sebald, is rather sentimental and kitschy.
This was also Böll's first novel -- he was 33 when it was written. But it is already a mature work, and does not have the feel of juvenalia about it. Some of the writting is both beautiful and insightful.
The second half is devoted to finding meaning in the 'Catastrophe' (that is, the events in Germany just after Capitulation), and appeals to Böll's deeply held Catholicism. Böll was from the south of Germany.
Though I am not a Catholic, I have occasionally found Catholic literature to be quite moving. Georges Bernanos' Diary of a Country Priest, for example, breaths this same atmosphere of poignant hopefullness in the midst of despair that one finds to some degree in The Silent Angel. But as I am not a believer, I found Böll's faith somewhat distressing, and that detracted (for me) from the value of this book.
Nonetheless, this is definitely a book worth reading. I haven't read any other Böll yet (though I did read a short story of his in German once back in graduate school - in German for Reading), and plan to read Billiards in the next week or two.