There're 4 books(paperback), 2 books from each series. It seems Series I has several editions. Currently it's hard to find brand new copies, but you may be able to find some used copies at Amazon.co.uk and Amazon.com. Rarely you can find also on eBay.

Spellbinder 1 Book 1 Riana's World   by Mark Shirrefs and John Thomson Find and Buy at Amazon.co.uk Find and Buy at Amazon.com
	Publisher: Hodder Children's Books ISBN: 0340667370 Year Published: 1996 Price: Unknown
	Publisher: Hodder Children's Books Australia ISBN: 0733601332 Year Published: 1995 Price: $10.95
	Publisher: Starlight ISBN: Unknown Year Published: Unknown Price: Unknown
Spellbinder 1 Book 2 Paul's World   by Mark Shirrefs and John Thomson Find and Buy at Amazon.co.uk Find and Buy at Amazon.com
	Publisher: Hodder Children's Books ISBN: 0340667389 Year Published: 1996 Price: Unknown
	Publisher: Hodder Children's Books Australia ISBN: Unknown Year Published: 1995 Price: Unknown
	Publisher: Starlight ISBN: 0733601340 Year Published: 1995 Price: $10.95
Spellbinder 2 Book 1 Land of the Dragon Lord   by Mark Shirrefs and John Thomson
	Publisher: Hodder Children's Books Australia ISBN: 0733604145 Year Published: 1997 Price: $10.95
Spellbinder 2 Book 2 Lost Between Worlds   by Mark Shirrefs and John Thomson
	Publisher: Hodder Children's Books Australia ISBN: 0733604153 Year Published: 1997 Price: $11.95

Note

These information may have some mistakes. I do not have any responsibility. So please confirm before purchase.

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