Using the full library of D&D 3.5e books, which magic items, if any, can grant a player a +1 to an ability score?
There are no items that grant a +1 enhancement bonus to any ability score. I have looked.
Ultimately, a +1 enhancement bonus to an ability score would welcome a lot of min-maxing in ability score point-buy (and, to a lesser extent, rolling or arrays). It would allow you to fairly trivially take an odd number for a score (saving points, especially since the following even number will have increased marginal cost for scores of 13 or higher), and get that last point with a cheap magic item. In short, such items allow characters to pretty easily boost their less-valuable ability scores, mitigating any weaknesses those lower scores would have caused (those scores important enough to get enhancement bonuses anyway, probably a primary score along with Constitution, don’t really change much since characters tend to try to get the largest enhancement bonus possible from them). It also lowers the effective “cost” of ability score requirements, for those who aren’t meeting them naturally (which I might buy would improve the game, but YMMV—Wizards, at least, didn’t seem to think so).
There are, however, the consumable manuals and tomes that, when read, grant a permanent inherent bonus to an ability score. Those are made with wish, and like wish itself, they can grant bonuses from +1 to +5.
But those inherent bonuses, as well as the ability score bumps every 4 levels, are far more valuable than the 1,000 gp of a hypothetical +1-enhancement bonus item, and thus are less “abusive” as alternatives to having the scores you need.
An extremely generous DM may allow master's third eyes
If a full library of 3.5 sources includes Wizards of the Coast online material, the Mind's Eye Web column "The Crystal Master (Prestige Class)" includes a new kind of psionic item, the master's third eye. The column on Psionic Items says
The crystal master knows how to make special third eyes called master's third eyes based on the gems she normally embeds. The following is the list of gems she can craft into a master's third eye. Only a crystal master can craft these master's third eyes, though they can be used by any character and do not count as an embedded gem for the crystal master. Crafting these master's third eyes follows the standard item creation rules for third eyes. If these master's third eyes lose their power for whatever reason, they revert back to a simple 10 gp gem.
Among master's third eyes are those that can grant an odd-numbered enhancement bonus to an ability score, with a master's third eye that grants a +1 enhancement bonus having a price of 2,000 gp; a +3 enhancement bonus a price of 18,000 gp; and and a +5 enhancement bonus a price of 50,0000 gp. These master's third eyes are in addition to those master's third eyes that can provide even-numbered enhancement bonuses of +2 and +4 at twice the price of a normal slotted enhancement bonus-granting magic items (hence 8,000 gp and 32,000 gp, respectively).
While the crystal master prestige class itself is updated in light of the 3.5 revision in the Mind's Eye Web column "Crystal Master Prestige Class (v.3.5)", the master's third eyes from the original column's Psionic Items section—that includes the quotation above—were not also updated, making them technically legal in a 3.5 campaign, albeit subject to "minor adjustments" by the DM. (For example, a 3.5 DM will likely rule that a master's third eye—like third eyes found in the Magic Item Compendium—occupies the face slot rather than being slotless (yet a creature could still only wear but one) as when third eyes were introduced in the Psionics Handbook, but the same DM may also halve a master's third eye price as it's no longer essentially slotless; also see the Dungeon Master's Guide (2003) on Why a Revision? (4)).
Further, it goes almost without saying that most DMs probably use the Expanded Psionics Handbook for their campaign's psionics needs rather than psionics from the original Psionics Handbook and the material that supported it. This makes it likely that anything having to do with pre-Expanded Psionics Handbook psionics—like the older Mind's Eye Web columns—is either ignored, banned, or considering obsolete, no matter how technically legal the material.
Thus in the typical 3.5 campaign don't count on master's third eyes being for sale at ye olde magick shoppe, and if you encounter a crystal master during your 3.5 adventures you probably shouldn't get your hopes up that she'll make you a master's third eye… or even know what it is you're talking about.
Otherwise, like this fine answer says, there just aren't any magic items that grant odd-numbered bonuses to ability scores unless they're inherent bonuses like those that can be gained from a manual of quickness and action (Dungeon Master's Guide 262) (27,500+ gp; 5 lbs.) and a tome of understanding (DMG 268) (27,500+ gp; 5 lbs.).
Note: You can read 3e co-designer Monte Cook's opinion on odd-numbered ability score bonus-granting magic items here. In short, he's not a fan.
Manuals and Tomes can add anywhere from +1 to +5 to an ability score as an inherent bonus - manuals for str, dex, and con, tomes for int, wis, and cha.
Additionally, Wishes can grant a +1 to a stat as an inherent bonus (up to +5 if multiple wishes used simultaneously). The Ring of Three Wishes, Luck Blades, Efreeti Bottles, and The Deck of Many Things can all give Wishes, although the later two are not reliable.
Finally, the Deck of Many Things can also give a +2 inherent bonus directly.