I can't think of any information relating directly to distiction of letters in the english Alphabet, so instead I pulled out some stuff on our corrolations between shapes and long/short term memory:
In four experiments, we examined the effect of pairing colors with either homogeneous or heterogeneous shapes on a short-term memory task. In Experiment 1, we found no differences in color memory for displays in which colors were each associated with different shapes, paired with individual homogeneous shapes, or paired with heterogeneous shapes. In contrast, in Experiment 2, we found that when participants were asked to remember the specific pairings of colors, memory was improved for heterogeneous-shape displays. The benefit for heterogeneous shapes appears to be memorial, rather than one that occurs at the time of encoding (Experiment 3) or retrieval (Experiment 4). The present study suggests that distinctive shapes can be used to help bind color associations in visual short-term memory.
However focusing more on shape I think this article by Ray Vellest, and particularly its section "Shapes", goes into a nice discussion of how logo's are designed to engage long term memory, and how by differntiating the type font, new meaning can be given to the letter.
This substantially answers your question by giving examples of how differentiating shapes, such as the type font, can make letters easier to remember.
Clearly, just a small change to the shape of the letter makes it have a completley different meaning. In this case, the sort of letter you would find in your text book, and the one you immeadiately associate with McDonalds
Apllying this to fonts would involve similar logic, but I'm hoping someone can pull in some information more directly related to the Alphabet then shapes.
Besides that, I hoped this helped you out.
Delvenne JF1, Dent K. (2008). Distinctive Shapes Benefit Short-Term Memory for colour association, but not for colour. http://www.ncbi.nlm.nih.gov/pubmed/18717388
Ray Vellest (2012). THe pychology of logo Design. http://www.webdesignerdepot.com/2012/08/the-psychology-of-logo-design/