This document describes using linear regression for top-N recommendation at Mendeley's social network products. It discusses using SLIM (Sparse Linear Methods), but finding it slow for large datasets. The author instead uses regularized linear regression trained with SGD. This approach improves over nearest neighbors on Mendeley's dataset of 5M documents, 1M users, and 140M interactions. Key-value side information like readership counts and document keywords are included. The method offers recommendations to both active and anonymous users in a way that is computationally efficient, customizable, and transparent compared to black box approaches.