Extended Public Key 's mechanism misinterpreting in 'Mastering Bitcoin' by Andreas I am a newbie and start exploring from the 'Mastering Bitcoin' book. The figure below to me is a little inaccurate, I think the arrow from HMAC-SHA256 should point right to the child public key. Am I right??
I think the arrow from HMAC-SHA256 should point right to the child
  public key. Am I right??


No, there are 3 derivation functions defined in BIP32 - Hierarchical Deterministic Wallets


  
  Private parent key → private child key
  Public parent key → public child key
  Private parent key → public child key
  


The diagram above illustrates 

1. Private parent key → private child key

Perhaps you're confusing this for 2. Public parent key → public child key. Another potential confusion is that this is not for a hardened child key but a normal child key, which uses the parent public key point(kpar), combined with the parent chain code cpar and the index i. These are combined and hashed to I as follows:

I = HMAC-SHA512(Key = cpar, Data = serP(point(kpar)) || ser32(i))

I is split as shown in the diagram, where the left 256 bits IL are combined with the parent private key kpar to produce the child private key ki:

ki = parse256(IL) + kpar (mod n)

IR is the new child's chain code at that index i, and the child public key is then calculated like normal using elliptic curve secp256k1 from the child private key itself.