I get asked this questions all the time, and my answer is always the same: It should be as long as it needs to be to clearly and intelligently get your point across.
Here are some general guidelines to keep in mind and get you started.
Page count vs. word count. Writing your manuscript in MS Word is much different that publishing your book in an eBook or 6 x 9 print format. Page count is irrelevant because the number of pages will change depending on the format. For this reason, I suggest you think and plan in terms of word count instead.
Readers lose interest after 30K words. I’ve conducted a number of polls on this topic throughout the years and I’ve learned that readers then to lose interest. In fact, just the visual impact of the size of the book can be a turn off. It can look like too much work or that it might take too long or have the perception that it’s full of fluff.
Readers may be disappointed by less than 25K words. This leaves the reader feeling ripped off, depending on the price. This gives the perception of a brochure rather than a book and nobody wants to pay for a brochure.
The sweet spot. Readers are more likely to enjoy and invest in a book that has a total word count between 25K and 32K. Visually, the book appears to be a manageable read and one that could be consumed in a matter of days rather than months.