Two studies, by Princeton psychologists, have found that using a hard-to-read font can lead to improved memory performance. They compared the retention of material set in Monotype Corsiva, Comic Sans Italicized and Haettenschweiler versus the same material set in Helvetica and Arial. From the research paper published in Cognition:
This study demonstrated that student retention of material across a wide range of subjects (science and humanities classes) and difficulty levels (regular, Honors and Advanced Placement) can be significantly improved in naturalistic settings by presenting reading material in a format that is slightly harder to read.
Fluency interventions are extremely cost-effective, and font manipulations could be easily integrated into new printed and electronic educational materials at no additional cost to teachers, school systems, or distributors. Moreover, fluency interventions do not require curriculum reform or interfere with teachersâ€™ classroom management or teaching styles.
I doubt that textbooks are in any danger of being typeset in crappy typefaces, but there’s probably some room for typeface variation that could improve learning. This technique is probably more relevant for handouts and classroom materials provided by teachers.