I'm not a beginner in music (6 years guitar playing) and I want to learn piano. The matter is there are a lot of materials for either absolute beginners or professionals showing how to improvise. I'm looking for any kind of help in order to learn writing/reading notes (because I play guitar using tabs only), understand and use intervals and scales and apply all there knowledge to play piano.
Like you, I started learning piano long after I had sung and played guitar for many years. I think there is no substitute for a few private lessons with a piano teacher to put you on the right path. Try to find a teacher who is sympathetic and flexible. You need to see if you can find someone who can do more than teach you rote lessons out of a method book (because, based on your guitar experience, you are not an absolute beginner to music.) If you could find a teacher who teaches both piano and guitar, then you could get help in applying your existing understanding of the guitar to the piano. Good luck.
I've gone the opposite way. I learned piano for a couple of years then tried my hand at guitar. I thought I wouldn't need any kind of beginner's books either, but a couple of months later saw me missing out on valuable techniques that would've made learning the instrument much easier. (Most of) guitar is... in the least offensive way possible, dumbed down, sheet-music-wise. Tabs are an example: they are useless anywhere else in music, and are really only a substitute for manuscript.
If you're only used to tabs, I'd recommend using a beginner's book. A slow and steady introduction into reading music is the easiest way to make quick progress with it. Scales, arpeggios and chords are completely different. You don't want to cut corners with an illusion that the knowledge is there, because, although it is, it really isn't applicable to both instruments. It goes as far as fingers: a 'first finger' on guitar isn't a 'first finger' on piano.
Had I the choice, I'd go back and learn guitar from scratch, as my playing ability plateaued after realising I was at the mercy of 'proper' guitarists' critical analysis. So I'd recommend you dig out a basic kiddie's book and start with the basics.
...That being said, being able to play one will make learning the other much easier. Learning violin gave me the real knowledge of sheet music I needed for piano, and the knowledge of scales and chords and keys got me ready for guitar. They will come in handy, but only when the core of the instrument (unique for every instrument) is learned.