While I am unable to recommend particular entry-level titles on the subject, I can recommend a look at one of the publications by the Royal Netherlands Institute of South East Asian and Caribbean Studies on the matter (headed by the aptly named Prof. Dr. Oostindie): the Journal of the Humanities and Social Sciences of Southeast Asia. It's a multidisciplinary academic journal with articles written in English as well as Dutch, with a focus on all things Indonesia with its publications available for free reading and download. The linked webpage has a search tool on the right-hand side which you can use to browse all its publications since 1853 for keywords. Not only might the actual content be of interest, but the journal, like many academic journals, also contains book reviews.

EDIT: I also would be doing the subreddit a disservice if I forgot to point you towards the excellent Booklist, which contains literature on Pre-Colonial Southeast Asia as well.

Are you looking for Srivijaya period Sumatra or the period after that?