Get a library card!

There are so many books at the library—not only is it more economical than buying a bunch of books, but it also gives you a chance to "try before you buy". San Francisco's Public Library system is awesome, with convenient neighborhood branches, no overdue fines (!), and relatively easy holds/interlibrary loans. You might wait until your kid is ~1 year old or so, since babies tend to chew books...

Board Books for Babies/Young Toddlers

Before your baby is about 2 years old, you'll want to mostly get "board books", which are harder to destroy

Books for 2-4 Year Olds

Here are some books we liked:

Read Aloud Chapter Books (4+ Year Olds)

Early school-aged kids are just starting to read on their own, but love hearing more complex stories when read to.

...also consider graphic novels, which are a great way for early readers to get into reading; Adam got obsessed with a bunch of Minecraft-themed ones