I think this is important just to get the idioms down that you otherwise only get on the job. Could pick almost any big active C project. I used to do this and I’d see some construct I didn’t understand and consult my K&R book to learn it. I’d do this in parallel with a book though for sure, K&R preferably. C is so basic that if you pursue it further it will be about learning the projects/companies way of using C.