Have you ever wondered why some books use -ise spellings , such as organisation, while others use the spelling organization? Usually, this is not merely an author’s whim but follows a recognized spelling style.
In British-style books many verbs that end in -ise can also be spelt with an -ize ending: for instance, organise and organize, recognise and recognize. Also, some nouns can be similarly spelt either way: organisation and organization. Whether you use -ise or -ize spellings in your writing is a matter of personal choice or maybe to follow the required style of the publisher or institution you are writing for, but for whatever reason the spelling used should be consistent throughout the document or publication. For instance, Oxford University Press prefers -ize spellings.
However, the house style of a publisher might accept both spellings in the case of a large book with individual chapters contributed by a variety of authors, especially if some are British and others are from the US or other areas of the world. If a large book contains a collection of papers that have been previously published separately, then a mix of spelling styles would likely be accepted as long as each chapter is consistent within itself. The same goes for other cases of variant spellings such as focused and focussed, and judgment and judgement.
Some people might think that -ize spellings are customary in the US, which would be the case, and are not used in Britain, which would not be the case. Some also might think that the use of -ize spellings in the UK is a recent innovation, but it is not as there have been cases of its use in nineteenth century English literature:
http://random-idea-english.blogspot.co.uk/2013/09/random-ise-novels-of-jane-austen-and.html
http://random-idea-english.blogspot.co.uk/2013/09/random-ise-works-of-george-eliot-ize.html
and even as far back as the fifteenth century according to the Oxford University Press:
http://blog.oxforddictionaries.com/2011/03/ize-or-ise/
There are a few verbs in British English (and usually in US English also) that can only be spelt with an -ise ending, such as advertise, chastise, comprise, compromise, exercise, improvise, premise, revise, surprise, and televise. This list is not exhaustive, but just gives a few examples.
There are also some verbs that always end in -yse in British English, but would be spelt ending in -yze in US English:
British / US
analyse / analyze
breathalyse / breathalyze
catalyse / catalyze
dialyse / dialyze
electrolyse / electrolyze
paralyse / paralyze
psychoanalyse / psychoanalyze.
Again, the above list is not exhaustive but just gives a few examples.
In summary, be consistent with your spelling patterns and refer to a good-quality dictionary.