Occult Slayer is great for this
The Occult Slayer’s Blank Thoughts class feature does sound ideal for your purposes: it’s preventing anyone from prying secrets directly from your brain, and it’s Extraordinary, so no one can shut it down or suppress it. Quite possibly one of the “hardest” forms of protection in the game.
If your mind is your fortress, make sure you can stuff as much in there as possible: Autohypnosis
I strongly recommend training Autohypnosis, if you can. That way you can memorize even very long texts, avoiding the need to keep a hard copy that could get stolen.
Broader Protection: Mind Blank
The spell mind blank is almost-perfect immunity to snooping, seeing as it covers all [Mind-Affecting] effects and Divination effects that try to learn about you, explicitly covering “all devices and spells that detect, influence, or read emotions or thoughts.” With mind blank up, you literally need worry only about things that say they work despite mind blank specifically; if it doesn’t mention mind blank by name, it won’t work. Even the know secrets salient divine ability doesn’t break it.
Mind blank’s only real vulnerabilities are dispel magic and antimagic field/Dead Magic Zones. If someone takes down your mind blank, particularly if you only prepared one for the day thanks to its 24-hour duration, you might be in trouble. Boosting your caster level is always good, as are things like a Ring of Counterspells keyed to greater dispel magic. Master Abjurer (Master Specialist option, Complete Mage) gets a hefty bonus to preventing their buffs from being dispelled, for example.
If you can get mind blank as a Supernatural Ability, it becomes immune to dispelling and you therefore only need worry about antimagic field or dead magic zones. The Dweormerkeeper1 prestige class from the Complete Divine web enhancement can turn any spell into a Supernatural Ability, for example; there may be other ways to get Su mind blank as well.
If you somehow get mind blank as an Extraordinary Ability or Salient Divine Ability, then it even works in an antimagic field. I doubt either is possible though. The Occult Slayer’s Blank Thoughts is better in these situations.
Vulnerability You Can’t Do Much About: Gods of Secrets
Between the two of these, all you need worry about are running afoul of deities’ portfolios. These can’t give the deity direct access to your thoughts, but they might discover things you’ve done, objects you’re hiding, and so on: any time something leaves your head and becomes a part of the external world, there’s probably a related deity who knows about it. Remember that there are gods of secrets out there; literally the act of keeping a secret itself informs the god about it.
1 Dweormerkeeper is easily in the top-10 most-powerful prestige classes in the game, and probably should be banned at the overwhelming majority of tables (in addition to being impossible to dispel, Supernatural Abilities also have no components – an Su wish, for example, does not cost XP).