Book preferred length
Most people think "longer is better". It actually isn't. At least not in the beginning. Most first time publishers prefer shorter than a hundred thousand words. When you look at famous books series and see that the first book was way shorter than the sequels, a lot of people may think that this is because the author was still working on their skills and couldn't make more, while this may be true with some authors there are others who do this on purpose.
The situation is that more pages are more expensive, and so making a three hundred and fifty page book first a completely new unknown author may be too risky an investment. And so longer does not actually equal better with publishers, and another thing that should be noted is that making a longer book runs risk of it being boring and less likely to sell well. The best thing is not to add filler but to try and finish everything right.
Of course, this may run the risk of having too short a book as well, but remember that a first time writer is better with not having a dragged out book. Instead, I'd personally try to aim above 50,000 and below 100,000 for your first one. After that I'd say you can go as high as you feel is right, all up to personal preference from then out.
You may also want to look into average page count for a book from the specific genre your book is going to be, for good measure. Especially if you're nervous.
That's all for now about book length, see you soon!