I would like to suggest suspense collection "Going Insane" by Tim Kizer.
[MODERATOR: The OP is the author.]
It contains "Intoxication" and "Hitchhiker."
From description:
"Intoxication"-
How do you force a confession out of a coworker trying to poison you? Do you kill those who dismiss your fears and believe you are paranoid? What do you do if you start questioning your own suspicions--and sanity--as you take the law into your hands?
In this disturbing tale of derangement, a young psychopathic woman is slipping into madness as she fights an enemy that may exist only in her imagination. She has to resort to desperate measures when she realizes that a gun, security cameras in her apartment, and constant vigilance will not be enough to survive.
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"Hitchhiker"-
When a serial killer hitches a ride one sunny day in a beautiful California valley, he does not suspect that he may have met his match, who is dead set to take another life. The battle of wits begins and only the most devious mind will survive.
Get it here for 99c:
http://www.amazon.com/dp/B004RJ3UCU