One of the great contributions of musicology over the past several decades is helping us see more clearly how composers in previous eras worked. Before the era of, say, Beethoven, music was never conceived as a permanent work that would stay in a single form for all eternity. It was put together for performance (or a few performances) and then it was off to the next work. Occasionally works would be revived, but that was more the exception than the rule. Playing works from previous eras was not a common practice until the mid-nineteenth century and is more a factor in our time than in any previous century.
This great work by the Dutch musicologist Jens Peter Larsen is a scholarly work rather than a book for a general reader, but it is still immensely interesting for anyone interested in understanding how this culturally irreplaceable work came into being, how it was performed and adapted the pieces to the forces Handel had available each time he performed the work. One of the interesting things we learn about the composer's work habits is that he wrote so rapidly and often carelessly ("Messiah" was put together in only twenty-three days) was due to his expectation that things would be worked out in rehearsing for the live performance. Errors could be fixed, pieces could be re-written for the singer, pieces could be cut, another could be composed and added if needed, and so forth.
Another thing that helped Handel write so quickly (it was his normal method of working) is that he often re-used (recycled is our word, I guess) earlier music. This was quite acceptable because, again, music was performed and then let go. It was expected that new music would be constantly written and that might include borrowing from one of your earlier works, or even re-working ideas from another composer. That was the method of the time and their views on copyright and what we call plagiarism were not theirs.
The chapters covering a survey of "Messiah" and changing versions are most interesting. It is also fascinating to look at the plates of the actual working copies of the score (even if they are very hard to read in this small format).
This book has had a tremendous influence on the way the oratorio has been performed since the early 1980s. Those wonderfully lively and clear performances you hear nowadays are partly the result of this fascinating book.