First, the decision to start designing embedded antennas in house is mainly one of cost. Contrary to common belief, EM simulation software, although helpful and insightful, requires an experienced user to benefit from it and is not absolutely mandatory, especially for the relatively simple planar PIFAs you describe. However, as you are aware, specific knowledge of how to design antennas and then how to successfully integrate them is mandatory.
On the question of cost, a dedicated employee can cost $100k + per year, especially when you consider benefits. If you are having a current employee take over the responsibility, then learning antenna design will take away from his/her normal responsibilities. It is not a straight forward science to most engineers that don't have previous experience.
Most small to medium sized companies I work with find outsourcing their antenna designs much less expensive as well as seamless to their product development activities. Taking on the antenna design may inhibit the smooth flow of your development, especially if problems arise late in the process.
To answer your question: 1) Planar Antennas for Wireless Communications" by Kin-Lu Wong (editor), Wiley & Sons 2003 is a very good book with lots of dimensioned drawings and results, 2) Can't beat Antenna Engineering Handbook (in its third edition now), by Richard C. Johnson, McGraw Hill 1993 for general antenna knowledge 3) The ARRL handbook is another good source for general antenna knowledge. Mine isn't handy just now or I would supply details. Just search for it on the Internet and it will take you to the ARRL site.
Good luck. If you need any help let me know -- I can do training as well as design.
an10a@cia-aa.com
