7th edition Call of Cthulhu rules give two mythos skill increase values for each mythos tome: CMI (initial reading) and CMF (full study). The general principle is clear: you gain a bit of knowledge from a basic skim of the book, and significantly more from a deep dive into the horrible content therein. What is not entirely clear from reading the rules is whether the CMI value is contained within (ie. is a subset of) the CMF value.
For example, suppose a tome has CMI +5 and CMF +12. Jim skims through the book and gets +5 to his Mythos skill. When he completes a full study, does he then gain another full 12 points of mythos skill (for a total gain of +17)? Or does he gain an additional 7 points (bringing his total points gained from that book to +12)?