I've got nothing for relationships, but here are four great parenting books by two very good economists. These aren't just great books about parenting, they're amongst my favorite economics books.

Bryan Caplan Selfish Reasons to Have More Kids The main thesis of this book is that parents are killing themselves trying to be perfect when they can actually enjoy themselves and have fun with their kids. The conclusions are drawn from fascinating research on twins that allows for teasing out the role of parenting vs genetics in children's outcomes.

Emily Oster Emily's three books (linked above) are an empirical assessment of specific parenting practices. Largely consistent with Caplan's work, some practices really do seem to have statistically significant effects, but in general the importance of those practices has been greatly inflated by poorly designed studies.