A brick of a book

I've done it. I finally finished Let the Galaxy Burn.

It was one hell of a fight. I wrestled with this brick of a book for almost four months. It has over 700 pages, densely packed with text, forming 38 separate short stories. It is certainly value for money. Most of the stories are collected from Inferno magazine, previous anthologies and White Dwarf, but some are brand new. It certainly isn't practical. I got a hernia carrying it round in my bag after I realised I'd be 134 if I only read it at home.

Now I have only 2 Black Library books to read, both of them a similar size, but rather than short stories a trilogy of novels. One is the Ultramarines omnibus and the other is the Last Chancers book. Any ideas on what I should read first?