Ads for "more books you might like, since you liked this one well enough to finish it" are good. The only problem is when the ads have a substantial page count; there are ebooks with barely 40,000 words of text followed by 20,000+ words of notes, ads, and excerpts from other novels; some readers feel cheated when they're told a book is "60,000 words long" and they discover that 1/3 of what they paid for is ads. See:
The Bloated eBook File @ Dear Author.
Other than that, people are generally happy with ads at the *end* of the book, where they can just stop reading if they're not interested. (Including me; I'm fine with ads & excerpts.) It's mostly the issue of paying to receive advertising that annoys people, with a smaller nuisance of "I thought this was a novel, not a novella and three intro chapters for other books."
A page or two of "also by this author" or even "by this other author, who writes similar stuff" (maybe you find someone else to cross-promote with) is fine.