Assuming you are talking about Psionic Powers, and not Spells(which the Psion cannot cast):
In the Expanded Psionics book (which is the official 3.5 Psionics book), in the Psion class description you can see this text under the "Discipline" section:
Discipline: Every psion must decide at 1st level which psionic discipline he will specialize in. Choosing a discipline provides a psion with access to the class skills associated with that discipline (see above), as well as the powers restricted to that discipline. However, choosing a discipline also means that the psion cannot learn powers that are restricted to other disciplines. He can't even use such powers by employing psionic items. (See Psionic Disciplines, below, for a brief description of each discipline. Chapter 5 of this book contains a list of powers available to all psions and lists of powers restricted to certain disciplines.)
Reading that I understand that you simply cannot learn any power that is restricted to another discipline.
In the next section, "Powers known" there is also this text:
Choose the powers known from the psion power list, or from the list of powers of your chosen discipline. You cannot choose powers from disciplines other than your chosen discipline. (Exception: The feats Expanded Knowledge, p. 46, and Epic Expanded Knowledge, p. 34, allow a psion to learn powers from the lists of other disciplines or even other classes.) A psion can manifest any power that has a power point cost equal to or lower than his manifester level.
Which confirms that you cannot use powers that are from other disciplines.
In the class table, this is the information for level 1:
- Power points per day: 2
- Powers known: 3
- Maximum power level known: 1st
You are probably reading an outdated book.
Also, just in case you are in doubt: a Psion's high Intelligence will grant her more Power points, but not more powers known.