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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)?

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Without being specifically notified otherwise, it's wise to assume that the CMF is an additional gain (it's what the text says and that's what the "+" in the table implies)

p174 : "If the reader’s Cthulhu Mythos skill is below the Mythos rating of the book, they gain Cthulhu Mythos points equal to the full study (CMF) number."

If you need more arguments, you'll find that the sum of CMI+CMF in the 7ed table corresponds pretty much to the total gain that was described in previous editions. For instance, Al Azif has a gain of 18 percentiles in 6ed, a CMI of 6 and a CMF of 12 in 7ed.

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    \$\begingroup\$ Both answered so far received are good and conclusive and if I could accept both I would. I’m opting for this one because the rules quotes seems marginally more on-point, and also (and especially) for pointing out the correlation between the combined CMI + CMF and the all-in-one value from earlier editions — which is a compelling argument. \$\endgroup\$ – Rick Lecoat Apr 22 at 7:22
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The first Full Reading that the investigator undertakes gives the full benefit

Referencing the 7th ed rulebook, it does not say anything about the initial reading affecting the full readings.

There is even an example given the backs up this point.

Harvey gets his hands on the Book of Eibon...

Book of Eibon: Sanity point loss 2D4. Cthulhu Mythos +3%/+8%, Full study 32 weeks. Mythos rating 33%.

Harvey begins reading the book in his home and then continues while on a long train journey... Harvey’s player makes the roll and is rewarded with +3% Cthulhu Mythos knowledge (the Cthulhu Mythos Initial—CMI number for the tome) ...

Later he decides to undertake a full study of the Book of Eibon... so he gains the +8% Cthulhu Mythos skill points for a full study (CMF)

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    \$\begingroup\$ Thanks Destruktor. I had read that example but I didn’t find it entirely unambiguous. ‘…he gains the +8% Cthulhu Mythos skill points for a full study’ could be taken to mean ‘…he is taken up to the full +8% gain for that book.’ Not that I’m doubting that you’re right, you understand. I’m just overthinking it. \$\endgroup\$ – Rick Lecoat Apr 22 at 7:18

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