Norske boeker er dyrE
Aarsak is not quite frequent. It s much better to say  grunn  if you mean  reason .
Especially = spesielt or SAERLIG

BRUKE penger

jeg har ikke bestemt aa spendere penger
That doesn t look like Norwegian though native speakers would understand you for sure
I d say: jeg har bestemt MEG FOR aa+

Eksperimentet er sluttet+ again: not quite Norwegian. A better way to say it is: det er slutt med eksperimentet

If you meant  my favorite writer  then it should be  min yndlingsforfatter .
Mange boeker (ikke mye!)
Aa skjoenne = to understand
Use  Jeg tror at+. Jeg syns at+ , if you want to express your opinion.