I know that Warlock can maximize and empower spell like abilities but I am having trouble finding in which book/page this is mentioned.
Complete Arcane, page 81. I'd give some more details to show that the Warlock is allowed to do this, but the description of Maximize Spell-Like Ability does the work for me - it uses the Warlock as an example:
For example, a 10th-level warlock’s maximized eldritch blast deals 36 points of damage three times per day.
and it also covers Empowering:
An empowered maximized spell-like ability gains the benefit of each feat separately (getting the maximum result plus one-half the normally rolled result).
As for why this doesn't affect caster level. It doesn't because it doesn't say that it does.