For an unknown author wanting to be published by a publishing house, word count is essential.
There are countless websites where agents explain where the "sweet spot" lies for a certain genre. For memoirs, as White Eagle has already stated, it lies around 65,000 words.
Does a memoir have to have 65,000 words? No, of course not.
- The better your book is,
- the more famous you are, and
- the more books have previously sold,
the more you can deviate from the sweet spot.
The reason for this is twofold. The first aspect is price. Thicker and heavier books are more costly to produce, transport, and store and therefore have a higher cover price. But readers are more willing to buy a book from an unknown author, if its price is at the lower end of the price spectrum. Think of a product you are unsure about. If it's cheap, you may try it, but if it's expensive, you'll rather buy something you are sure won't disappoint you.
The other aspect is reader expectation. Certain kinds of stories usually take a certain "time" or word count to narrate. A book that appears too short looks like it won't satisfy, while a book that appears too long looks like it will bore you. You can check this yourself. If you think of a certain kind of novel, you will probably not only imagine a certain kind of story, but also how slim or hefty the book will feel in your hand. You don't expect a Young Adult contemporary novel to look and feel like a fantasy epic, and if it does, most readers will hesitate because they think the book won't deliver.
So do make sure your book falls within +/- 10% of the ideal length, if you can. If you cannot, see what your beta readers have to say.