The two books we currently recommend are:
How do they compare?
The former is brief, hardcopy, with full colour photos, formatted to publishing standard, and written by a well-respected scything instructor in the UK.
The latter is voluminous, detailed, with annotated diagrams, in PDF (but you could print it), quite rough around the edges in terms of layout, and written by the Vido family, who have probably done more than anyone to promote and improve the tool and associated accessories and techniques in modern times. And it’s free.