We recommend that the tone of your manuscript should be formal, although preferably not stuffy. Formal yet readable by a normal person is the best tone for the majority of projects. There may be some books that will benefit from a more academic tone (such as an edited volume of conference proceedings), or from a less formal and more “friendly” tone (such as a book designed for some kind of non-specialist “outreach”).
Your writing should be clear and concise without being verbose. Furthermore, your choice of words should be appropriate to the audience for your work: a beginner’s guide will not need complicated words, as these would complicate the learning process for such an audience, whereas they would be entirely appropriate for a volume aimed at experts. Our editors will be only too happy to ask you to rephrase sections that are too wordy or otherwise ambiguous.
In general, be accessible in your language, and strive for quality writing. This is the most important rule.