My "sacred relics" are these:
A well-worn copy of Noad's "Solo Guitar Playing, Vol. 1", and a Ricordi edition of the Carcassi 25 Studies, edited by Guido Margaria. Both date from my high school studies in the 1970s. Both have lots of fingerings and lesson commentaries penciled in by my first CG teacher, Tom Runyan in Los Angeles. Tom is the one who introduced me to the 10-string, and the Carcassi in particular has several places where he changed things to make use of the additional basses.
Close runners-up in significance are Noad's books "The Renaissance Guitar" and "The Baroque Guitar", both received as gifts for my 18th birthday. These likewise have lots of markings in them. The "Renaissance" book is physically in terrible shape, it is held together with tape. I re-fingered many of the pieces for lute tuning, going back to the lute tabs while I was at University and changing things to reflect the original 3/F# tuning. Years later I gained a better appreciation for how skillful an editor Noad really was, the changes he made didn't really impact the overall impact or shape of the pieces. (SIDEBAR: I also have the "Classical" and "Romantic" books which complete the Noad anthology series, but neither one of those books ever got the degree of attention that the first two got from me.)
Fifth and last of the "sacred books" would be the Belwin-Mills "Complete Works of Barrios, Vol. 1" published in 1976 and edited by Richard Stover. This was Volume 1 of a four-volume series, later supplanted by the much bigger two-volume Stover edition. This particular copy is full of markings from my years of study with Stover directly, corrections of errors and jotted comments based on then-newly found manuscripts (which later went into the bigger set).
Salt Lake City, UT