First of all; the seller has way over priced Monette's books at scalping rates for these volumes. I happen to know that they have been for sale for years and have not sold because these are specialty books and the buyers are not the fools that the seller apparently thinks that they are. Simply because books are rare or out of print or the owners particular treasure is no justification for scalping prices. These volumes on a particular phase of pipe organ maintenance and construction are of no value to anyone not interested in Organbuilding other than some selfish bibliophile who would collect it for the sake of collecting it, have no interest in the contents and let it collect dust upon his shelves when others could use the information contained within. I own books that date back to 1516 that are not only out of print but also the only existing copy but the value is not thousands or millions of dollars but would you believe a hundred or less?
The volumes promise much but do not deliver all that one is lead to believe that they will deliver. Much of these information is already contained in such books as Dom Bedos famous treatise (Facteur d'orgues), Audsley, among others such as Marenholtz's Treatise on Scaling. These volumes would be more useful if accompanied by audio-visual resources to go with the text because some things cannot easily described, even by a Shakespeare, in words.
My advice to any potential buyer is to search for a more reasonably price seller. Before buying, borrow a copy of these volume(s) from a public library or through inter-library load to be sure this is something that will fullfil your needs and it will not end upon your shelf collecting dust unused. If you still want it and cannot find a copy---great things often come to he who waits long enough.