I'd like to learn the Chongqing dialect or chongqinghua. If anyone can help me to find resources like phrasebooks, differences with putonghua, or any article on this topic, I would be thankful.
As a form of Southwestern Mandarin, you can approach the Chongqing dialect with resources designed for Sichuanese in general. The English Wikipedia gives a lot of resources on "Si4cuan1hua4", including a good overview of the phonology, and a introduction to Sichuanese Pinyin. The Chinese Wikipedia gives a little more detail on the Chengdu-Chongqing dialect.
As for books, most will naturally be in Chinese. In English, there is the Lonely Planet China Phrasebook, which includes Sichuanese, but with a very idiosyncratic romanisation. Otherwise, there are only academic papers (e.g. Malmqvist's Syntax of Bound Forms in Sïch'uanese), although there are quite a few that mention Sichuanese or one of its constituent topolects.
Of the free papers that I was able to find in Chinese, there is this on the evolution of the checked tone from Middle Chinese through early Southwestern Mandarin to the modern Chongqing dialect.
There are naturally quite a few books about Sichuanese and the Chongqing dialect in Chinese, among them the 四川方言詞語考釋 (ISBN: 7806593713), the 四川方言词汇研究 (ISBN: 7500488165), both about vocabulary; and 四川方言与普通话 (ISBN unknown; published 1982) compares it to Standard Mandarin. Sichuanese-to-Chinese dictionaries include this one from 1987; there is one for the Chongqing dialect in particular (ISBN: 7562113378, apparently); also, one for the related Chengdu dialect from 1998, 成都方言词典 (ISBN: 7534334209 apparently), was one of the rather epic 现代汉语方言大词典 series. There is also a course, 四川方言会通 (ISBN: 7807521880), apparently published in 2008.
Elsewhere on the Internet, there are quite a few websites in English based on Chengdu Sichuanese rather than Chongqing Sichuanese. Among them, this Chengdu diary has an entry concentrating on dialect; there are a few guides to the phonology on gochengdoo.com; and there is an interesting Sichuanese primer on the blog Long Legged Fly. There is a now (in)famous video of a white American person speaking some Sichuanese here on YouTube; the Tudou version is here. Also some here on Tudou. Plus a few courses on the Chongqing dialect can be on fyan8.com.
This resource recommends learning Mandarin first, and then learning Chongqinghua using Mandarin. http://www.chinese-forums.com/index.php?/topic/35629-resources-for-learning-chongqinghua/
This post claims it's almost identical to Mandarin with different tones and the trend is for it to be rolled into Mandarin since it's so similar already. http://matthewsoren.blogspot.com/2009/06/chongqinghua-chongqing-dialect.html
Then Google stops giving interesting results. It seems like given it's intense similarities to Mandarin and obscurity in the west, your best bet to study it is actually to study it using Mandarin as a basis.