Whereas drooping LE slats increase camber which should increase lift, this is partially (in many cases wholly) offset by the fact that the effective angle of attack of the wing is also reduced by the drooping leading edge. Hence, the main effect of the drooping slats is to delay the stall both by by delaying the separation point through through energising the airflow over the wing as described above and by actually having reduced the effective angle of attack of the wing, combined with an increase of its cord.
Plain trailing edge flaps, on the other hand, do nothing for the stall characteristics (excepting Fowler and similar) but merely increase the CL for a given AoA.
Hence, the graph shown in your book, where LE slats extend the CL/AoA curve upwards, whereas TE flaps shift it to the left without significantly affecting the critical AoA.
B.