Eggs by Marilyn Singer, illustrated by Emma Stevenson
I did not review this book when I first read it. I checked it out, took it to school, and handed it straight to the first grade teacher. It is, bar none, the best book about eggs and nests I have ever seen for early elementary school children. Don't get me wrong: An Egg Is Quiet is great, and gorgeous, but Eggs has about 300 times more information - perfect for inquisitive little kids.
These few sentences on fertilization are worth the price of the book.
"It usually takes a male and a female to make babies. Females make the eggs. Males fertilize them with sperm. When birds, reptiles, mammals, insects and some amphibians mate, the eggs are fertilized inside the female."
Cut, dried. Enough information to explain the process, but not so much as to make most parents uncomfortable.
The rest of the book is just as well-written, and - oh, hey? have I mentioned the illustrations yet? Holy mackerel. And holy honeybee, and holy red admiral caterpillar, holy mayfly, alligator, octopus egg, cave swift nest...
Source notes, a glossary, a list of wildlife protection organizations and a "What you can do" section round out the back matter.
Personal bonus: the author acknowledges the assistance of many people I used to know at the American Museum of Natural History, including that great old bedbug Lou Sorkin. (Thanks for the baby coral tarantula, Lou!)