I can recommend an inexpensive index to the maps which is FREE in good bookshops and which describes the grid and the coverage of each map. Note there was a misprint on the back of the 1997 edition - Kingston on Hull is in square TA and not in square T. You may find Bartholomew maps on sale. These have essentially the same co-ordinate system as OS but a modified reference system. They have references which omit the two letter square of the OS system but give eastings and northings in metres from the OS false origin. You will find references to three different norths in the margin of OS maps.
True north is just that - a direct line to the North Pole and the Earth's spin axis.
Grid North is as defined by the grid lines on the map which cannot match true north because the map is a flat representation of a curved surface. For practical purposes though true north is not very relevant and we are only interested in grid and magnetic north, the difference between which varies uniformly across the country with only small anomalies. This difference is small compared with that in many parts of the world but not quite small enough to be ignored - it was 4 degrees west of grid in Wales in 1996, decreasing by a degree every 8 years. Whittaker's Almanac documents this variation in Britain, is published annually and should be found in the reference section of any public library. Neither term is actually printed in the margins of the OS maps.