I use apps like the free dictionary and dictionary.com . By creating an account you can save the words you have searched.

If I own the book, I write the definitions on the pages where I encounter the words and sometimes even do drawings.

It makes for very slow reading though. Hopefully I'll eventually reach a point where I don't feel the need to look up the words.

The annoying part of reading in a second language for me is that I usually don't know if the word is known by a majority of people. Reading in my native language I may stumble on a word I don't know and think : I'm positively certain that over 99% of people don't know that word so I don't see why I should.