The now defunct Mainline Modeler Magazine did a series of books called the Best of Series, which are now out of print. Some of you might remember the magazine and as you know, the pages were filled with great information and plans as well as scratchbuilding and superdetailing articles.
I have Freight Cars Volume 1 and Passenger Cars Volume 1 and I will tell you these are must haves for any modeler who wants to increase their skills and level of detail in their work. There are several soft cover books ranging from freight cars to bridges.
The articles are a collection of historical documentation with plans and in-depth how-to’s that take you step-by-step through the project.

Although these books are out of print, you can still find used copies on web sites such as amazon.com as well as craigslist.com.