I can't find the book definitions on the net at the moment but perhaps we need to remind ourselves that:-
Transition altitude is the altitude (in the "vicinity" of an aerodrome) at or below which the vertical position of the a/c will be expressed (to ATC) in terms of an altitude based on QNH.
Transition level is the Flight Level at or above which the vertical position of an a/c will be expressed in terms of a Flight Level (based on 1013.25 mb)
The bit between the Transition Altitude and the Transition Level is the Transition Layer. When flying in the Transition Layer the vertical position of the aircraft is expressed in terms of a Flight Level when climbing or an Altitude when descending.
The way I teach the computation of the Transition Level is to imagine you climb (with QNH set) and level off at the Transition Altitude (although fixed for a particular aerodrome it does vary across the UK - eg 3,000 ft for aerodromes outside controlled airspace, 4000 ft at EMA/BHX.....etc as published). Whilst flying level you then set 1013.25 mb. If you wind on the pressure the altimeter indication will increase, if you wind off the pressure it will decrease (by approx 30 ft per mb). Lets say the altimeter now reads 3,400 - the TL will then be FL 35 (ie the next 500 ft level above your current "level"). Alternatively lets say the altimeter now reads 2,300 - the TL would then be FL 25.
I hope this helps.