Depends on your objective:
- if you have an old (really old) book and want to conserve it this way -> tif or pdf with facsimile-like images embedded : readable on pc/tablet/10"ereader. Chance is good that you can still read it in a few years
- if you want to read immediately -> cleaning and fast screening for errors -> searchable pdf : readable on pc/tablet/10"ereader (if not to avoid - 6")
- if you are the "editor" type of person with a sense for aesthetics -> cleaning, thorough editing, layouting and publishing to epub or whatever : readable on all current ereaders. What happens in a few years?
This sequence also represents a scale of effort
Klaus