CALI, the Center for Computer-Assisted Legal Instruction, 
offers hundreds of online lessons and tutorials, many of which focus on writing in legal genres.
Because legal writing organization, logical structure, and vocabulary can differ dramatically from other academic and professional writing, this focus on writing specifically for legal purposes is particularly useful. For example, one 45-minute tutorial focuses on how to write Issue Statements for memos and briefs. (Registration is probably required in order for the link to this activity to work, but here is a snapshot of what the opening page looks like.)
Ebooks under Creative Commons license are also available.
To use the site, it is necessary to register, but registration is free for members of CALI-associated institutions. To check to see if you qualify for free registration, check the list that CALI provides to see how to get your registration code. Each institution has a different means of getting a registration code to you, but links and contact information are provided.