A work of this nature is indistinguishable from any normal book (cf. Manual of Bodily Health). 

Any single character who reads a tome of clear thought will be able to practice mental exercises which will increase his or her Intelligence by 1 point. 

Reading a work of this nature takes 48 hours time over 6 days, and immediately thereafter the book disappears. 

The reader must begin a program of concentration and mental disciplines within 1 week of reading the tome. 

After 1 full month of such exercise, Intelligence goes up. 

If psionics are employed in the campaign and the character is previously a non-psionic, another check may be allowed. 

The knowledge gained from reading the work can never be recorded or articulated. 

Any further perusal of a tome of clear thought will be of no benefit to the character.

Special Note. All magical books, librams, manuals, tomes, etc. appear to be "normal" works of arcane lore. Each is completely indistinguishable from the other by visual examination of the outer parts or by detection for magic aura. 

Bard characters will have normal chances of finding out the nature of such writings, as will an identify spell from a magic-user. 

Otherwise, only a wish will be useful in typing a magical writing, i.e. alter reality, commune, contact higher planes, limited wish, true seeing, true sight, and other spells or powers are useless. 

A wish will reveal general contents of a book, telling what characteristics or class is most affected (not necessarily benefited) by the work. It requires a second wish to determine exact contents. 

After being perused by a character, most of these magical works will vanish forever, but those which are non-beneficial to the reader will typically be attached to the character, and he or she will be unable to be rid of it. 

If the work benefits another alignment of character, the possessor is geased to conceal and guard the writing. 

As Dungeon Master you should use your judgment and imagination as to exactly how these items will be treated, using the rules herein as parameters.