Which is a certified/authentic guide to entering Jhnanas? By far I have came across only Ajahn Brahms book which gives detailed instructions on entering Jhnanas.
If I google for any other instruction manual I get this link.
I don't doubt Ajahn Brahm at all but I havent read any reviews or blogs or internet postings of anyone who has entered Jhnanas through the instruction set of AB.
The link seems to be talking on similar lines but different language.
Is there a certified, peer reviewed instruction manual to entering Jnanas?
Can you provide any other instruction manual link preferably if you have experienced it.
First, without being explicitly jhana,  to get the citta into samadhi, like here
https://suttacentral.net/an10.2/en/sujato
http://obo.genaud.net/dhamma-vinaya/ati/sn/05_mv/sn05.55.040.than.ati.htm#p1
  there is, before, 'sukhaṃ vediyati'', meaning pleasant vedana or ''sukkha''. 
Before the pleasant vedana, there is ''kāyo passambhati'', which is a tranquil body. Before, the  tranquil body, there is ''pītimanassa'' which is mano having piti. Before mano has piti, there is ''Pamuditassa'', which is the experience of joy (of being moral and rejecting thought of delight on kama and ill will, which makes you keep the 5 precepts and all those rules about morality). Before the joy, there is a lack of regrets.

This stuff about  eagerness for renunciation, seeing drawbacks, and nirodha is the discriminating factor between a success and a failure to reach sotapanna or arhanthood through the jhanas, 
http://obo.genaud.net/dhamma-vinaya/ati/an/04_fours/an04.178.than.ati.htm#p1

Like the buddha said about him, https://suttacentral.net/an9.41/en/sujato, he claimed that renunciation of sensuality was good, but his citta was not '' confident'' in it. And as usual, the citta is not inclined towards renunciation due to a lack of seeing a danger in sensuality. Once he saw this danger, he managed to get the jhanas. 

Now explicitly for the jhanas. First there is a whole vagga about jhanas
http://obo.genaud.net/backmatter/indexes/sutta/sn/03_kv/idx_34_jhanasamyutta.htm

second, the usual way to enter the first jhana is to be ''aloof from sense-desires, aloof from unprofitable states,'' from the typical formulation http://www.buddha-vacana.org/formulae/4jhana.html#1-cl
 and the buddha says that samasamadhi is precisely those jhanas
http://www.buddha-vacana.org/sutta/samyutta/maha/sn45-008.html#sammasamadhi

so the situation where somebody is ''aloof from sense-desires, aloof from unprofitable states,''  should stem form the joy and lack of regrets from being moral and seeing drawbacks of cravings for and delight in sensuality.