Tome of Secrets was level 6, but since it takes over 5 days a scout it wasn't worth keeping. The 2kp a day from the ToS was nice. I also had the Spire Library and Tournament Arena. They are very late game and I really only built them for the avatars and got a few ranking points while I had them, but never leveled them. I'm not really into ranking that much, though it is fun and part of the game. There are a few others I've contemplated selling, but haven't felt the need yet. Strangely enough, the Dwarvan Bulwark is one. Since I have a lot of high level armories, it doesn't boost my training size near as much as my Shrooms AW.
AW choice changes as you move through the game. There are several I never got that others love. Many don't like the ToS, but I got a lot of hammers from it until the Dragons chapter. I never built the Bell Spire (human) and only built the Watchtower Ruins and BTG recently. So, don't go by what I say here when choosing your AWs, please. Study what your needs are and will be. They are powerful buildings. Though, the Fire Phoenix is the best military building of them all and it's not an AW.