Sources for Old Books

Hello everyone,

I would like to hear your thoughts from those of you whose research has taken them beyond Zacatecas and Jalisco and into Spain. I am currently researching a couple of ancestor families, Velasco and Alvarez de Asturias, and exhausted PARES research. I came across what appear to be old manuscripts written from 1695 to late 1700’s. These books have a wealth of information in regards to family trees, but they sometimes contradict each other. For example, there may be entire generations mentioned in one book and not the other. Or the parent of the same individual is completely different from one text to the other. I don’t expect you guys to read the entire chapters because there is just too much info and difficult to follow without putting in time.

I guess my only question, how reliable are these old manuscripts as a source of information and research?

Here are some of the links to these books.

Regarding Velasco Family (Example: Dia Sanchez de Velasco missing in one text and not the other)
Pages- After page 481

https://books.google.com/books?id=V8wWAAAAQAAJ&pg=RA1-PR1&lpg=RA1-PR1&d…

Pages- 597

https://books.google.com/books?id=i-FDAAAAcAAJ&pg=PA597&lpg=PA597&dq=In…

Pages- 322

https://books.google.com/books?id=1utDAAAAcAAJ&pg=PA322&lpg=PA322&dq=Mo…

Regarding: Alvarez de Asturias (Example:Parent of Don Ordoño is Fruela II in one text and Don Ramiro II in another)
Pages- 279

https://books.google.com/books?id=63gazWSJHL0C&pg=PA279&lpg=PA279&dq=or…

Pages- 50-53

https://books.google.com/books?id=xypddg8RrUAC&pg=PA51&lpg=PA51&dq=cond…