You book could have some explanations and this fourum is
not the worst place to start
(unfortunatelly my time is limited,
I'll try to answer specific questions). Btw, I just looked at the drawing
again and I noticed the diode D1 to the right of relay coil. I didn't notice
it before because of way it's drawn.
There is one more thing I did't notice last time (always in a hurry...)
it's the R5 pot. As far as I remember pin7 of the 555 chip is open
collector output used for discharging of capacitor (it simply shorts it
to gnd). Problem is that if this happens while R5 is set to it's lowest
resistance, that transistor which is inside 555 would blow (and the Q2
transistr as well). I would add small "safety" resistor such as 1k in
series with the R5.
There is no current limiting resistor for IR diode either (sender) but
that might be ok since it should work with low duty cycle.