Are there any resources for learners of Chinese dialects that are close to Mandarin? I've lived in Henan provice for around 8 years and have never seen any texts for learners of the Henanese dialect. There are considerable variations in dialect within Henan between people from different cities and even different districts within the same city. I am able to understand much of what is spoken, but can't figure out which tones are being used because they are so inconsistent. So the only way I know of learning Henanese is to memorize the pronunciation of phrases.

Henanese is much closer to Mandarin than dialects like Cantonese, Wenzhounese, etc. That may explain the lack of teaching material.

I know of foreigners who have picked up the local dialect, but I'm more of a visual learner. It would be far more efficient to look at a written guide. Are there any such resources for Henanese or other Chinese dialects?
There is not much of this available even at bookstores within China. There is even much less for non-background speakers in English.

I personally have been lucky enough to get a book from Peking University Press titled "Practical Suzhou Dialect" which also includes English. I tried searching online however this appears to be a one-off. However, you could try contacting one of the major universities or even going to a university in Henan to see if they have resources available.

There is also this book listed on Amazon Henan dialect of Luoshan but this is not a text book. I have tried seeing if I could find the contents of this online, but only found a site listing an index of what is inside. 

I also suggest asking on Chinese social media sites to see if anyone can help.