From the SRD (emphasis mine):
Although the alchemist doesn’t actually cast spells, he does have a
formulae list that determines what extracts he can create
...
An extract is “cast” by drinking it, as if imbibing a potion—the effects
of an extract exactly duplicate the spell upon which its formula is
based
...
An alchemist can also add formulae to his book just like a wizard adds
spells to his spellbook, using the same costs and time requirements.
An alchemist can study a wizard’s spellbook to learn any formula that
is equivalent to a spell the spellbook contains. A wizard, however,
cannot learn spells from a formula book.
Alchemists cannot learn spells, they learn formulae, that are based on actual spells, but are not spells. Thus, if a wizard’s spellbook contains a spell that is the equivalent of a existing formula, you can study the spellbook to add that formula to your own formula book (essentially recreating the formula from the spell). However, the wizard cannot do the opposite.
Also, as a side comment, since alchemist extracts work like potions that must be ingested, makes sense that most formulae mimic spells that are either of personal range, buffs or healing, due that method of application. Attack and debuff spells could not be properly applied with that method (just imagine the consecuences of pouring a fireball down your throat), so it would not make sense that the alchemist could learn any wizard spell.
