Reliable channel or method for self-study of Psychology
Psychology can be said to be an empirical science which is built up out of the elements of experience (Avelin, 1933).

Assuming Psychology in general is empirical, its principles can be investigated by experiments, e.g., through the gold standard of clinical research, namely randomized controlled clinical trials (RCTs). Such studies are generally considered to be reliable, yet they are not always accurate, as they are still prone to all sorts of troubles, to name a few see Fig. 1.

All these potential sources of error can lead to false conclusions and hence different studies may yield conflicting evidence. As such, the 'truth' may be out there, but often it is represented in the scientific community by consensus, rather than fact. Different people may adhere to different beliefs, and rely their views on a select number of results, rather than the consensus. As such, even reputable textbooks may convey a biased message. 

To search for the book of truth, as you seem to do, is in vain. It hasn't existed, does not exist, and will not exist. In fact, the best sources are meta-studies that compare the available data critically and come to a consensus, rather then stating them as facts.    


Fig. 1. source: Sessley &amp; Imrey (2015)  

References
- Avelin, Nature (1933); 132: 841–3
- Sessley &amp; Imrey, Anesthesia-analgesia (2015); 121(4): 134-42