Of particular relevance, we found evidence for a strong coupling over time between cognitive abilities . . . and reading

Well this is an interesting summary that I have never seen anywhere. I love reading and am convinced that there is a beneficial relationship between enthusiastic reading and cognitive capability. However, I have never seen any studies that empirically support that assumption. But apparently I have been looking in the wrong places. This summary from Uncoupling of Reading and IQ Over Time: Empirical Evidence for a Definition of Dyslexia by Emilio Ferrer, Bennett A. Shaywitz, John M. Holahan, Karen Marchione, and Sally E. Shaywitz is pretty categorical.

In previous research (Ferrer & McArdle, 2004), we found a strong association between the development of cognitive ability and changes in academic achievement (i.e., academic knowledge and quantitative abilities) during childhood and
adolescence. We described these interconnections as coupled developmental sequences in which levels of one variable were
positively related to changes in time in the other variable. Of particular relevance, we found evidence for a strong coupling over time between cognitive abilities (i.e., Full Scale, Nonverbal, and Verbal IQ on the Wechsler Intelligence Scale for Children–Revised, or WISC-R) and reading (i.e., Letter-Word Identification, Word Attack, and Passage comprehension reading scales from the Woodcock-Johnson Psycho-Educational Battery, or WJ) for individuals in Grades 1 to 12 (Ferrer et al., 2007). Such couplings represented influences from, for example, cognition in a given year to positive changes in reading the following year. Results from those analyses indicated that (a) there was a positive dynamic relation between reading and cognitive ability from Grades 1 to 12; (b) this dynamic relation was symbiotic, with each process influencing the other over time; and (c) the mutual dynamics of reading and cognition appeared to be strongest during Grades 1 to 3, less strong during 4th to 8th grade, and weakest from 9th to 12th grade. Studies of adults, too, have found that cognitive ability (Verbal IQ) predicts reading accuracy (Berninger, Abbott, Thomson, & Raskind, 2001).

I am reading this to say that reading and IQ are reinforcing and that the biggest impact is in the earliest years.

There is also this from Diet, Parental Behavior, and Preschool Can Boost Children’s IQ by Anna Mikulak.

Interventions focused on interactive reading — teaching parents how to engage their children while reading with them — were found to raise children’s IQ by over 6 points. These interventions do not seem to have an effect for children over 4 years old, suggesting that the interventions may accelerate language development, which, in turn, boosts IQ.