Reddit reviews The Bare Naked Book

I was just about to recommend those! Who Has What?: All About Girls' Bodies and Boys' Bodies and The Bare Naked Book are good introductions to body parts, and It's Not the Stork discusses where babies come from as well.

The Who Has What book already linked is great, my kid also has the Bare Naked Book which she likes to look at sometimes (she's 3 in February).

Don't get squeamish about the kid knowing the word penis or whatever. It's just a word, and it's a correct word for a body part that like 50% of people have. My kid talks about them sometimes like this: "he has a penis" (regarding her dad when he changes for the shower). Or sometimes "When I'm big I'll get a penis" because at the moment she really wants to be able to pee standing up lol.

They're gonna think words for genitalia are hilarious for quite some time so just get used to that :) it's better to not think the words are taboo though.

Check out this one, we like it a lot.