Heh, and I'll quote yor earlier post:
"UA states that standard spell books can contain "up to 36 cantrips, 24 spells of 1st-3rd level, up to 16 spells of 4th-6th level, or up to 8 spells of 7th-9th level" (p. 79). Travelling spell books, on the other hand, have the following capacities: "nine cantrips; six spells of 1st, 2nd, and/or 3rd level; four spells of 4th, 5th and/or 6th level; or two spells of 7th, 8th and/or 9th level" (p. 79). "
A spell book can hold 8 spells of 7th+ level. If 5 are inscribed, that means it is at 5/8ths capacity, so it could indeed contain 6 additional spells of 4th-6th level, or 9 of 1st-3rd level.
Cheers,
Gary