I think it's a great book for a medium or large-sized company where there is some task sharing. If each admin is very specialized and can't handle being a buffer for anyone else, it's going to be harder to organize.
Sole/lone admins will find a good chunk of the book hard to implement just because then it's harder to manage interrupt vs project time. Overworked admins may get some level of relief, but if there's more work than time, it almost makes it worse to stare at your constantly growing lists because of the importance placed on arranging them.
I don't know of any other books that feel like they're speaking to "us" instead of more general time management and I don't regret buying it, even if I find it's hard to get going with certain parts.