what book? You can start signing with your baby whenever you want. Choose signs that you'd use often & come naturally, like "milk," "more," "all done/finished" etc. signing consistently with your baby will allow him to soak it in the way he soaks in spoken language. Typically, a baby is ready to sign back to you before s/he is ready to speak.
In our case, dd signed "more" at 8 months, & spoke her first 2 words at 9 months. Not a big difference but she had gross motor developmental delays & needed a year of PT. while she was focused on motor development, her expressive language slowed down greatly. Signing saved us from a lot of frustration! Again, when she was 2.5, she developed chronic fluid in her ears & had a moderate hearing loss for 3 months before she got tubes. Again, signing to the rescue
Ok, blathering on about us (insert blushing smilie). Sorry about that. Definitely sign to him in naturally occuring contexts (nursing, diaper changes, etc.). The more you sign the more comfortable you'll become signing & the easier it will be for all of you!
In our case, dd signed "more" at 8 months, & spoke her first 2 words at 9 months. Not a big difference but she had gross motor developmental delays & needed a year of PT. while she was focused on motor development, her expressive language slowed down greatly. Signing saved us from a lot of frustration! Again, when she was 2.5, she developed chronic fluid in her ears & had a moderate hearing loss for 3 months before she got tubes. Again, signing to the rescue

Ok, blathering on about us (insert blushing smilie). Sorry about that. Definitely sign to him in naturally occuring contexts (nursing, diaper changes, etc.). The more you sign the more comfortable you'll become signing & the easier it will be for all of you!