The book has a narrow focus: "The great majority of the process engineer's work is strictly with organic chemicals". Whilst that is clearly Mr Irwin's experience, it hasn't been mine, or that of the majority of process engineers I know.
But Erwin is a practitioner, and his approach is practical. He recognizes upfront that even the programs he supplies with the book on disc "have not been put through an exhaustive beta test", so he says that the "quest of this book, to correct the program through your good ability". He is not implying that we should forget the IChemE's Guidance on Use of Computers
So I'd recommend this book to those who wish to use VB to carry out design tasks (especially those in the petrochemical industry), but they should be aware that by the time you have written and tested your programme, other methodologies might well have given you an answer a great deal more quickly, unless the programme is to be reused many times.
I have some Excel spreadsheets that I use to do hydraulic calculations, which have a bit of VB in them. I went to all of the trouble of having them third-part validated because I knew I would be using it many hundreds of times in subsequent years. It's just not worth the trouble for a one-off task.