I have searched the rule book (1995) but did not found a concrete information - does he/she only gain paradox?
A botched spell is "blown" and generates Paradox. I have taken the time to look at some resources and you are right. Nowhere in the Corebook, Storyteller's companion, Book of Shadows or Book of Madness (assuming pre-Revised edition) does it say that anything other than Paradox happens.
As written, it's just Paradox being gained and possibly a backlash occurs
Many tables I know of assume a houserule (possibly not realising it's not written anywhere) that while a successful magickal effect works and failed one does not, botched spell will work wrong.
Book of Madness in sections that deal with Paradox encourages people to relate Paradox Backlash to the effect being cast that caused the roll. In those cases it is advisable to warp the effect in a creative and nonsensical way that is also harmful to the caster (or his surroundings). Book of Shadows recommends accumulating small Paradox (1-5 point, but more commonly 1-2 points) without rolling for backlash. That said, Book of Shadows seems to assume that every botched roll generates enough Paradox for a Backlash roll to be justified - and that checks out with vulgar with witnesses and high-level vulgar without witnesses.
However, as with a great many things in Mage 2nd edition, what precisely happens when you botch a roll is up to the group to imagine. The practice shows that a lot of aspects of the play are left vague, either by omission or deliberate decision. As long as it makes sense to you and to the group, anything that you agree on can and will happen. Which in a meta way is exactly in the spirit of Mage.