THIS is the Best Book on Language Learning I've Ever Read: HERE'S WHAT IT SAYS
Here are some tricks for making rapid progress in learning one, two or three languages at the same time. The information I'm sharing is from The Loom of Language by Frederic Bodmer written in 1941. It's the best book I've ever read on learning a language and change the way I studied.
The Loom of Language: An Approach to the Mastery of Many Languages by Frederic Bodmer edited by Lancelot Hogben